While ARCH is being kinda special, I don't see PMS specifying that it doesn't need to be described alike other USE_EXPANDs, using profiles/desc. Unless I'm mistaken, we should add proper descriptions there.
PMS reference: https://projects.gentoo.org/pms/6/pms.html#x1-350004.4 desc/ This directory contains files analogous to use.desc for the various USE_EXPAND variables. Each file in it is named <varname>.desc, where <varname> is the variable name, in lowercase, whose possible values the file describes. The format of each file is as for use.desc, described in section 4.4.3. The USE_EXPAND name is not included as a prefix here. So literally, variables from USE_EXPAND_UNPREFIXED don't have a file there.
Ok, thanks for clarifying. That works for me.