Profiles use.desc and use.local.desc files
The use.desc file contains a list of all global non-expanded USE flags,
together with a short description. This file should not be modified without
discussion on the gentoo-dev list.
The metadata.xml file on each ebuild category contains a list of the local
USE flags, together with short description for the package in the said category.
More information about the metadata.xml file can be found
here.
use.local.desc used to be the default location for all the local useflags.
However, this file is now an autogenerated aggregation of all the metadata.xml files
so you must never edit it directly.
Having a small number of packages using identically named local USE flags is
allowed. If the number starts to grow substantially, it may be worth proposing
that the flag becomes a global — see
Local and global USE flags.
All non-expand flags must be listed in exactly one of these files.
Expanded flags are listed in desc/${prefix}.desc, where ${prefix} is the
name of the environment variable to be expanded in lowercase. Instead of listing the
complete flag only the flag's suffix, i.e. the environment variable's possible values are
listed.