/etc/portage/package.use

/etc/portage/package.use contains USE flags for individual packages. This is useful for USE flags that shouldn't be used globally (e.g. test, doc), and flags that are specific to a single package.

Note that can either be a single file or a directory containing per-package USE flags.

Format
One DEPEND atom per line with space-delimited USE flags, comment lines begin with #

See version specifier for information on DEPEND atom syntax.