/etc/portage/package.use

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

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

 * Portage manpage