In a review comment on https://codereview.chromium.org/2919853002
msramek@ said (in reaction to my suggestion to over-generalize the description line, which I agree was too far):

I'm convinced that saying "embedded on" is better than no string, but I'm not sure if it's precise in all cases.

Technically, nothing mandates that the primary pattern and secondary pattern are
the requesting and embedding origin. And in fact, it wasn't always that way (see
https://docs.google.com/spreadsheets/d/1Y0gFR5evM410O-CjuG3h-3aW3PQ9XiUE4-zYF...
).

However, given that we do not support manually adding double-keyed exceptions,
the only thing to add a double-keyed exception is through a permission prompt,
and in those cases primary and secondary pattern should always have their
standard meaning."

The purpose of this issue is to make note of this thought and follow-up on it. msramek@ feel free to wontFix it or drive it further (as you prefer). I'm just trying to be helpful by moving it out of the CL comment (where I worried that it would be lost).