@dsberry @nden : I'm saying that in common parlance, "coordinate system" does *not* always mean the same thing.  If I go up and down the hallway and ask different astronomers  what "coordinate systems" are (I actually just did that experiment), I get answers ranging "You mean like Cartesian or polar?" to "The way you describe objects in a catalog" to "It depends on the context".

Put more in the context of this proposal's language, some people of "coordinate system" as meaning the *frame*, while others think of it as a collection of frames (what you mean here), while yet others are thinking of specific points in a frame ("this catalog are in the ICRS coordinate system").

All I'm saying is that not worth worrying about which is "right", but rather just pick a phrasing that doesn't have this language ambiguity. That said, it may just be there's no other better word for it...