Ken and I had a disagreement in the editorial committee, and I'd like to resolve it in the UTC.
From my prospective, "compatibility characters" includes those characters that we wouldn't have encoded in Unicode except that we had to. Examples include characters like the Angstrom character that we needed to roundtrip with another standard, but also characters that didn't come from any other standard, but were the price to pay for avoiding other problems.
However, considerations of interoperability with other standards and systems [my bold] often require
that such compatibility characters be included in the Unicode Standard.
Conceptually, compatibility characters are characters that would not have been encoded in
the Unicode Standard except for compatibility and round-trip convertibility with other
I think the broader interpretation is the more useful, and more reflective of the purpose of calling these things "compatibility characters", and so would prefer that we use that interpretation in the text.