/etc/portage/package.use

contains USE flags for individual packages. This is useful for USE flags that should not be used globally (e.g.  or   USE flags), and for local flags that are specific to a single packages.

Example
For more details see the Handbook.

Format

 * One DEPEND atom per line with space-delimited USE flags (see version specifier for information on DEPEND atom syntax).
 * Comment lines begin with  (hash).

External resources

 * https://packages.gentoo.org/useflags/ - USE flags on Gentoo Packages Database
 * Portage manpage
 * http://blog.cafarelli.fr/2016/02/setting-use_expand-flags-in-package-use/