NativeWidgetAura is used for all Widgets in classic ChromeOS ash. On Windows and Linux desktop platforms, DesktopNativeWidgetAura is primarily used, but NativeWidgetAura can also be occasionally used. However, when running as a mus client, I don't think it makes sense to ever use NativeWidgetAura, since all Widgets should be backed by mus, and so should use NativeWidgetMus.