According to the NSAccessibilityConstants.h header file, these are the actual return types for all the NSAccessibilityAttributes (specified in the comments).

/* Standard attributes
 */
APPKIT_EXTERN NSString *const NSAccessibilityRoleAttribute;		//(NSString *) - type, non-localized (e.g. radioButton)
APPKIT_EXTERN NSString *const NSAccessibilityRoleDescriptionAttribute;	//(NSString *) - user readable role (e.g. "radio button")
APPKIT_EXTERN NSString *const NSAccessibilitySubroleAttribute;		//(NSString *) - type, non-localized (e.g. closeButton)
APPKIT_EXTERN NSString *const NSAccessibilityHelpAttribute;		//(NSString *) - instance description (e.g. a tool tip)
APPKIT_EXTERN NSString *const NSAccessibilityValueAttribute;		//(id)         - element's value
APPKIT_EXTERN NSString *const NSAccessibilityMinValueAttribute;		//(id)         - element's min value
APPKIT_EXTERN NSString *const NSAccessibilityMaxValueAttribute;		//(id)         - element's max value
APPKIT_EXTERN NSString *const NSAccessibilityEnabledAttribute;		//(NSNumber *) - (boolValue) responds to user?
APPKIT_EXTERN NSString *const NSAccessibilityFocusedAttribute;		//(NSNumber *) - (boolValue) has keyboard focus?
APPKIT_EXTERN NSString *const NSAccessibilityParentAttribute;		//(id)         - element containing you
APPKIT_EXTERN NSString *const NSAccessibilityChildrenAttribute;		//(NSArray *)  - elements you contain
APPKIT_EXTERN NSString *const NSAccessibilityWindowAttribute;		//(id)         - UIElement for the containing window
APPKIT_EXTERN NSString *const NSAccessibilityTopLevelUIElementAttribute;	//(id)         - UIElement for the containing top level element
APPKIT_EXTERN NSString *const NSAccessibilitySelectedChildrenAttribute;	//(NSArray *)  - child elements which are selected
APPKIT_EXTERN NSString *const NSAccessibilityVisibleChildrenAttribute;	//(NSArray *)  - child elements which are visible
APPKIT_EXTERN NSString *const NSAccessibilityPositionAttribute;		//(NSValue *)  - (pointValue) position in screen coords
APPKIT_EXTERN NSString *const NSAccessibilitySizeAttribute;		//(NSValue *)  - (sizeValue) size
APPKIT_EXTERN NSString *const NSAccessibilityContentsAttribute;		//(NSArray *)  - main elements
APPKIT_EXTERN NSString *const NSAccessibilityTitleAttribute;		//(NSString *) - visible text (e.g. of a push button)
APPKIT_EXTERN NSString *const NSAccessibilityDescriptionAttribute;	//(NSString *) - instance description
APPKIT_EXTERN NSString *const NSAccessibilityShownMenuAttribute;		//(id)         - menu being displayed
APPKIT_EXTERN NSString *const NSAccessibilityValueDescriptionAttribute	//(NSString *)  - text description of value
NS_AVAILABLE_MAC(10_5);
APPKIT_EXTERN NSString *const NSAccessibilitySharedFocusElementsAttribute NS_AVAILABLE_MAC(10_10);  //(NSArray *)  - elements that share focus


/* Misc attributes
 */
APPKIT_EXTERN NSString *const NSAccessibilityPreviousContentsAttribute;	//(NSArray *)  - main elements
APPKIT_EXTERN NSString *const NSAccessibilityNextContentsAttribute;	//(NSArray *)  - main elements
APPKIT_EXTERN NSString *const NSAccessibilityHeaderAttribute;		//(id)         - UIElement for header.
APPKIT_EXTERN NSString *const NSAccessibilityEditedAttribute;		//(NSNumber *) - (boolValue) is it dirty?
APPKIT_EXTERN NSString *const NSAccessibilityTabsAttribute;		//(NSArray *)  - UIElements for tabs
APPKIT_EXTERN NSString *const NSAccessibilityHorizontalScrollBarAttribute;//(id)       - UIElement for the horizontal scroller
APPKIT_EXTERN NSString *const NSAccessibilityVerticalScrollBarAttribute;//(id)         - UIElement for the vertical scroller
APPKIT_EXTERN NSString *const NSAccessibilityOverflowButtonAttribute;	//(id)         - UIElement for overflow
APPKIT_EXTERN NSString *const NSAccessibilityIncrementButtonAttribute;	//(id)         - UIElement for increment
APPKIT_EXTERN NSString *const NSAccessibilityDecrementButtonAttribute;	//(id)         - UIElement for decrement
APPKIT_EXTERN NSString *const NSAccessibilityFilenameAttribute;		//(NSString *) - filename
APPKIT_EXTERN NSString *const NSAccessibilityExpandedAttribute;		//(NSNumber *) - (boolValue) is expanded?
APPKIT_EXTERN NSString *const NSAccessibilitySelectedAttribute;		//(NSNumber *) - (boolValue) is selected?
APPKIT_EXTERN NSString *const NSAccessibilitySplittersAttribute;	//(NSArray *)  - UIElements for splitters
APPKIT_EXTERN NSString *const NSAccessibilityDocumentAttribute;		//(NSString *) - url as string - for open document
APPKIT_EXTERN NSString *const NSAccessibilityActivationPointAttribute;           //(NSValue *)  - (pointValue)

APPKIT_EXTERN NSString *const NSAccessibilityURLAttribute;		//(NSURL *)    - url
APPKIT_EXTERN NSString *const NSAccessibilityIndexAttribute;		//(NSNumber *)  - (intValue)

APPKIT_EXTERN NSString *const NSAccessibilityRowCountAttribute		//(NSNumber *)  - (intValue) number of rows
NS_AVAILABLE_MAC(10_5);
APPKIT_EXTERN NSString *const NSAccessibilityColumnCountAttribute	//(NSNumber *)  - (intValue) number of columns
NS_AVAILABLE_MAC(10_5);
APPKIT_EXTERN NSString *const NSAccessibilityOrderedByRowAttribute	//(NSNumber *)  - (boolValue) is ordered by row?
NS_AVAILABLE_MAC(10_5);

APPKIT_EXTERN NSString *const NSAccessibilityWarningValueAttribute	//(id)  - warning value of a level indicator, typically a number
NS_AVAILABLE_MAC(10_6);
APPKIT_EXTERN NSString *const NSAccessibilityCriticalValueAttribute	//(id)  - critical value of a level indicator, typically a number
NS_AVAILABLE_MAC(10_6);
APPKIT_EXTERN NSString *const NSAccessibilityPlaceholderValueAttribute	//(NSString *)  - placeholder value of a control such as a text field
NS_AVAILABLE_MAC(10_6);
APPKIT_EXTERN NSString *const NSAccessibilityContainsProtectedContentAttribute NS_AVAILABLE_MAC(10_9);   // (NSNumber *) - (boolValue) contains protected content?
APPKIT_EXTERN NSString *const NSAccessibilityAlternateUIVisibleAttribute NS_AVAILABLE_MAC(10_10);  //(NSNumber *) - (boolValue)
APPKIT_EXTERN NSString *const NSAccessibilityRequiredAttribute NS_AVAILABLE_MAC(10_12);  //(NSNumber *) - (boolValue) whether a form field is required to have content for successful submission of the form


/* Linkage attributes
 */
APPKIT_EXTERN NSString *const NSAccessibilityTitleUIElementAttribute;		//(id)	      - UIElement for the title
APPKIT_EXTERN NSString *const NSAccessibilityServesAsTitleForUIElementsAttribute; //(NSArray *) - UIElements this titles
APPKIT_EXTERN NSString *const NSAccessibilityLinkedUIElementsAttribute;		//(NSArray *) - corresponding UIElements


/* Text-specific attributes
 */
APPKIT_EXTERN NSString *const NSAccessibilitySelectedTextAttribute;		//(NSString *) - selected text
APPKIT_EXTERN NSString *const NSAccessibilitySelectedTextRangeAttribute;	//(NSValue *)  - (rangeValue) range of selected text
APPKIT_EXTERN NSString *const NSAccessibilityNumberOfCharactersAttribute;	//(NSNumber *) - number of characters
APPKIT_EXTERN NSString *const NSAccessibilityVisibleCharacterRangeAttribute;	//(NSValue *)  - (rangeValue) range of visible text
APPKIT_EXTERN NSString *const NSAccessibilitySharedTextUIElementsAttribute;	//(NSArray *)  - text views sharing text
APPKIT_EXTERN NSString *const NSAccessibilitySharedCharacterRangeAttribute;	//(NSValue *)  - (rangeValue) part of shared text in this view
APPKIT_EXTERN NSString *const NSAccessibilityInsertionPointLineNumberAttribute;	//(NSNumber *) - line# containing caret
APPKIT_EXTERN NSString *const NSAccessibilitySelectedTextRangesAttribute	//(NSArray<NSValue *> *) - array of NSValue (rangeValue) ranges of selected text
NS_AVAILABLE_MAC(10_5);


/* Parameterized text-specific attributes
 */
APPKIT_EXTERN NSString *const NSAccessibilityLineForIndexParameterizedAttribute;		//(NSNumber *) - line# for char index; param:(NSNumber *)
APPKIT_EXTERN NSString *const NSAccessibilityRangeForLineParameterizedAttribute;		//(NSValue *)  - (rangeValue) range of line; param:(NSNumber *)
APPKIT_EXTERN NSString *const NSAccessibilityStringForRangeParameterizedAttribute;	//(NSString *) - substring; param:(NSValue * - rangeValue)
APPKIT_EXTERN NSString *const NSAccessibilityRangeForPositionParameterizedAttribute;	//(NSValue *)  - (rangeValue) composed char range; param:(NSValue * - pointValue)
APPKIT_EXTERN NSString *const NSAccessibilityRangeForIndexParameterizedAttribute;	//(NSValue *)  - (rangeValue) composed char range; param:(NSNumber *)
APPKIT_EXTERN NSString *const NSAccessibilityBoundsForRangeParameterizedAttribute;	//(NSValue *)  - (rectValue) bounds of text; param:(NSValue * - rangeValue)
APPKIT_EXTERN NSString *const NSAccessibilityRTFForRangeParameterizedAttribute;		//(NSData *)   - rtf for text; param:(NSValue * - rangeValue)
APPKIT_EXTERN NSString *const NSAccessibilityStyleRangeForIndexParameterizedAttribute;	//(NSValue *)  - (rangeValue) extent of style run; param:(NSNumber *)
APPKIT_EXTERN NSString *const NSAccessibilityAttributedStringForRangeParameterizedAttribute; //(NSAttributedString *) - does _not_ use attributes from Appkit/AttributedString.h


/* Text attributed string attributes and constants
 */
APPKIT_EXTERN NSString *const NSAccessibilityFontTextAttribute;			//(NSDictionary *)  - NSAccessibilityFontXXXKey's
APPKIT_EXTERN NSString *const NSAccessibilityForegroundColorTextAttribute;	//CGColorRef
APPKIT_EXTERN NSString *const NSAccessibilityBackgroundColorTextAttribute;	//CGColorRef
APPKIT_EXTERN NSString *const NSAccessibilityUnderlineColorTextAttribute;	//CGColorRef
APPKIT_EXTERN NSString *const NSAccessibilityStrikethroughColorTextAttribute;	//CGColorRef
APPKIT_EXTERN NSString *const NSAccessibilityUnderlineTextAttribute;		//(NSNumber *)	    - underline style
APPKIT_EXTERN NSString *const NSAccessibilitySuperscriptTextAttribute;		//(NSNumber *)	    - superscript>0, subscript<0
APPKIT_EXTERN NSString *const NSAccessibilityStrikethroughTextAttribute;		//(NSNumber *)	    - (boolValue)
APPKIT_EXTERN NSString *const NSAccessibilityShadowTextAttribute;		//(NSNumber *)	    - (boolValue)
APPKIT_EXTERN NSString *const NSAccessibilityAttachmentTextAttribute;		//id - corresponding element
APPKIT_EXTERN NSString *const NSAccessibilityLinkTextAttribute;			//id - corresponding element
APPKIT_EXTERN NSString *const NSAccessibilityAutocorrectedTextAttribute NS_AVAILABLE_MAC(10_7);		//(NSNumber *)	    - (boolValue)
APPKIT_EXTERN NSString *const NSAccessibilityTextAlignmentAttribute NS_AVAILABLE_MAC(10_12);		//(NSNumber *) - (NSTextAlignment)

/* Textual list attributes and constants. Examples: unordered or ordered lists in a document.
 */
APPKIT_EXTERN NSString *const NSAccessibilityListItemPrefixTextAttribute    NS_AVAILABLE_MAC(10_11);    // NSAttributedString, the prepended string of the list item. If the string is a common unicode character (e.g. a bullet •), return that unicode character. For lists with images before the text, return a reasonable label of the image.
APPKIT_EXTERN NSString *const NSAccessibilityListItemIndexTextAttribute     NS_AVAILABLE_MAC(10_11);    // NSNumber, integerValue of the line index. Each list item increments the index, even for unordered lists. The first item should have index 0.
APPKIT_EXTERN NSString *const NSAccessibilityListItemLevelTextAttribute     NS_AVAILABLE_MAC(10_11);    // NSNumber, integerValue of the indent level. Each sublist increments the level. The first item should have level 0.

/*
 About MisspelledText attributes for attribute strings:
 
 NSAccessibilityMisspelledTextAttribute was the original attribute to indicate misspelled text. In OS X 10.4, the Cocoa text system added support for NSAccessibilityMarkedMisspelledTextAttribute, which was used to indicate a word that was visibly marked as misspelled (for example, with a red squiggle underneath); the original MisspelledText attribute could also be used on text that was not visibly marked as mispelled (for example, a misspelled word that was currently being edited by the user).
 
 Typically, a screen reader only wants to vocalize what a sighted user could see, and so the MarkedMisspelledText attribute was adopted by VoiceOver to provide feedback to the user about misspelled text. In OS X 10.9, VoiceOver has entirely stopped using the original MisspelledText attribute, and now only checks for MarkedMisspelledText.
 
 When implementing accessibility for a custom text-editing engine, you should generally provide the MarkedMisspelledText attribute in order to support VoiceOver, especially in OS X 10.9 and later. You may optionally also support the MisspelledText attribute for compatibility with other accessibility clients.
 */
APPKIT_EXTERN NSString *const NSAccessibilityMisspelledTextAttribute;		//(NSNumber *)	    - (boolValue)
APPKIT_EXTERN NSString *const NSAccessibilityMarkedMisspelledTextAttribute NS_AVAILABLE_MAC(10_4);	//(NSNumber *) - (boolValue)

APPKIT_EXTERN NSString *const NSAccessibilityFontNameKey;	// required
APPKIT_EXTERN NSString *const NSAccessibilityFontFamilyKey;	// optional
APPKIT_EXTERN NSString *const NSAccessibilityVisibleNameKey;	// optional
APPKIT_EXTERN NSString *const NSAccessibilityFontSizeKey;	// required


/* Window-specific attributes
 */
APPKIT_EXTERN NSString *const NSAccessibilityMainAttribute;		//(NSNumber *) - (boolValue) is it the main window?
APPKIT_EXTERN NSString *const NSAccessibilityMinimizedAttribute;	//(NSNumber *) - (boolValue) is window minimized?
APPKIT_EXTERN NSString *const NSAccessibilityCloseButtonAttribute;	//(id) - UIElement for close box (or nil)
APPKIT_EXTERN NSString *const NSAccessibilityZoomButtonAttribute;	//(id) - UIElement for zoom box (or nil)
APPKIT_EXTERN NSString *const NSAccessibilityMinimizeButtonAttribute;	//(id) - UIElement for miniaturize box (or nil)
APPKIT_EXTERN NSString *const NSAccessibilityToolbarButtonAttribute;	//(id) - UIElement for toolbar box (or nil)
APPKIT_EXTERN NSString *const NSAccessibilityProxyAttribute;		//(id) - UIElement for title's icon (or nil)
APPKIT_EXTERN NSString *const NSAccessibilityGrowAreaAttribute;		//(id) - UIElement for grow box (or nil)
APPKIT_EXTERN NSString *const NSAccessibilityModalAttribute;		//(NSNumber *) - (boolValue) is the window modal
APPKIT_EXTERN NSString *const NSAccessibilityDefaultButtonAttribute;	//(id) - UIElement for default button
APPKIT_EXTERN NSString *const NSAccessibilityCancelButtonAttribute;	//(id) - UIElement for cancel button
APPKIT_EXTERN NSString *const NSAccessibilityFullScreenButtonAttribute NS_AVAILABLE_MAC(10_7); //(id) - UIElement for full screen button (or nil)

/* Application-specific attributes
 */
APPKIT_EXTERN NSString *const NSAccessibilityMenuBarAttribute;		//(id)         - UIElement for the menu bar
APPKIT_EXTERN NSString *const NSAccessibilityWindowsAttribute;		//(NSArray *)  - UIElements for the windows
APPKIT_EXTERN NSString *const NSAccessibilityFrontmostAttribute;	//(NSNumber *) - (boolValue) is the app active?
APPKIT_EXTERN NSString *const NSAccessibilityHiddenAttribute;		//(NSNumber *) - (boolValue) is the app hidden?
APPKIT_EXTERN NSString *const NSAccessibilityMainWindowAttribute;	//(id)         - UIElement for the main window.
APPKIT_EXTERN NSString *const NSAccessibilityFocusedWindowAttribute;	//(id)         - UIElement for the key window.
APPKIT_EXTERN NSString *const NSAccessibilityFocusedUIElementAttribute;	//(id)         - Currently focused UIElement.
APPKIT_EXTERN NSString *const NSAccessibilityExtrasMenuBarAttribute NS_AVAILABLE_MAC(10_8);    //(id)         - UIElement for the application extras menu bar.

typedef NS_ENUM(NSInteger, NSAccessibilityOrientation) {
    NSAccessibilityOrientationUnknown = 0,
    NSAccessibilityOrientationVertical = 1,
    NSAccessibilityOrientationHorizontal = 2,
} NS_ENUM_AVAILABLE_MAC(10_10);

APPKIT_EXTERN NSString *const NSAccessibilityOrientationAttribute;	//(NSString *) - NSAccessibilityXXXOrientationValue
APPKIT_EXTERN NSString *const NSAccessibilityVerticalOrientationValue;
APPKIT_EXTERN NSString *const NSAccessibilityHorizontalOrientationValue;
APPKIT_EXTERN NSString *const NSAccessibilityUnknownOrientationValue;


APPKIT_EXTERN NSString *const NSAccessibilityColumnTitlesAttribute;	//(NSArray *)  - UIElements for titles

APPKIT_EXTERN NSString *const NSAccessibilitySearchButtonAttribute;	//(id)         - UIElement for search field search btn
APPKIT_EXTERN NSString *const NSAccessibilitySearchMenuAttribute;	//(id)         - UIElement for search field menu
APPKIT_EXTERN NSString *const NSAccessibilityClearButtonAttribute;	//(id)         - UIElement for search field clear btn


/* Table/outline view attributes
 */
APPKIT_EXTERN NSString *const NSAccessibilityRowsAttribute;		//(NSArray *)  - UIElements for rows
APPKIT_EXTERN NSString *const NSAccessibilityVisibleRowsAttribute;	//(NSArray *)  - UIElements for visible rows
APPKIT_EXTERN NSString *const NSAccessibilitySelectedRowsAttribute;	//(NSArray *)  - UIElements for selected rows
APPKIT_EXTERN NSString *const NSAccessibilityColumnsAttribute;		//(NSArray *)  - UIElements for columns
APPKIT_EXTERN NSString *const NSAccessibilityVisibleColumnsAttribute;	//(NSArray *)  - UIElements for visible columns
APPKIT_EXTERN NSString *const NSAccessibilitySelectedColumnsAttribute;	//(NSArray *)  - UIElements for selected columns
APPKIT_EXTERN NSString *const NSAccessibilitySortDirectionAttribute;	//(NSString *) - see sort direction values below

/* Cell-based table attributes
 */
APPKIT_EXTERN NSString *const NSAccessibilitySelectedCellsAttribute NS_AVAILABLE_MAC(10_6);	//(NSArray *)  - UIElements for selected cells
APPKIT_EXTERN NSString *const NSAccessibilityVisibleCellsAttribute NS_AVAILABLE_MAC(10_6);	//(NSArray *)  - UIElements for visible cells
APPKIT_EXTERN NSString *const NSAccessibilityRowHeaderUIElementsAttribute NS_AVAILABLE_MAC(10_6);	//(NSArray *)  - UIElements for row headers
APPKIT_EXTERN NSString *const NSAccessibilityColumnHeaderUIElementsAttribute NS_AVAILABLE_MAC(10_6);	//(NSArray *)  - UIElements for column headers

/* Cell-based table parameterized attributes.  The parameter for this attribute is an NSArray containing two NSNumbers, the first NSNumber specifies the column index, the second NSNumber specifies the row index.
 */
APPKIT_EXTERN NSString *const NSAccessibilityCellForColumnAndRowParameterizedAttribute NS_AVAILABLE_MAC(10_6);  // (id) - UIElement for cell at specified row and column

/* Cell attributes.  The index range contains both the starting index, and the index span in a table.
 */
APPKIT_EXTERN NSString *const NSAccessibilityRowIndexRangeAttribute NS_AVAILABLE_MAC(10_6);	//(NSValue *)  - (rangeValue) location and row span
APPKIT_EXTERN NSString *const NSAccessibilityColumnIndexRangeAttribute NS_AVAILABLE_MAC(10_6);   //(NSValue *)  - (rangeValue) location and column span

/* Layout area attributes
 */
APPKIT_EXTERN NSString *const NSAccessibilityHorizontalUnitsAttribute NS_AVAILABLE_MAC(10_6);   //(NSString *) - see ruler unit values below
APPKIT_EXTERN NSString *const NSAccessibilityVerticalUnitsAttribute NS_AVAILABLE_MAC(10_6);	    //(NSString *) - see ruler unit values below
APPKIT_EXTERN NSString *const NSAccessibilityHorizontalUnitDescriptionAttribute NS_AVAILABLE_MAC(10_6); //(NSString *)
APPKIT_EXTERN NSString *const NSAccessibilityVerticalUnitDescriptionAttribute NS_AVAILABLE_MAC(10_6);    //(NSString *)

/* Layout area parameterized attributes
 */
APPKIT_EXTERN NSString *const NSAccessibilityLayoutPointForScreenPointParameterizedAttribute NS_AVAILABLE_MAC(10_6); //(NSValue *)  - (pointValue); param:(NSValue * - pointValue)
APPKIT_EXTERN NSString *const NSAccessibilityLayoutSizeForScreenSizeParameterizedAttribute NS_AVAILABLE_MAC(10_6); //(NSValue *)  - (sizeValue); param:(NSValue * - sizeValue)
APPKIT_EXTERN NSString *const NSAccessibilityScreenPointForLayoutPointParameterizedAttribute NS_AVAILABLE_MAC(10_6); //(NSValue *)  - (pointValue); param:(NSValue * - pointValue)
APPKIT_EXTERN NSString *const NSAccessibilityScreenSizeForLayoutSizeParameterizedAttribute NS_AVAILABLE_MAC(10_6); //(NSValue *)  - (sizeValue); param:(NSValue * - sizeValue)

/* Layout item attributes
 */
APPKIT_EXTERN NSString *const NSAccessibilityHandlesAttribute NS_AVAILABLE_MAC(10_6); //(NSArray *)  - UIElements for handles


Make sure the attributes we have implemented in AXPlatformNodeCocoa match up with these return types.