U.S. vs. Canadian English keyboard

There's no difference except for the flag icon. It is presumably there so that Canadians don't have to be looking at the US flag all the time to type their language. The same holds for ABC, which is the same as US but no flag at all, for the rest of the world.

Unfortunately Apple does not document anywhere the dozens of different software keyboards/input sources which it provides.

The orginal question is for MacOS, but it may be useful for some to know that in iOS the act of using the particular keyboard while typing causes the auto-corrections and auto-suggestions to respect regional differences in spelling. Typing “color” while using Canadian English will result in a correction, while the same will happen if you type “colour” while using the US English keyboard. Likewise, the voice dictation feature will assume you want the regional spellings corresponding to your chosen input language.

Apple documents most hardware keyboards at:

• https://support.apple.com/en-us/HT201794

It has both French Canadian as well as English