/etc/portage/package.use

 provides a more fine grained per-package control of USE flags than the  line of.

The file or directory will override settings coming from the   line of.

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).

Automatically generated content
has the  option enabled by default (see ), so it can generate  settings as necessary to satisfy dependencies.

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/
 * Cleaning /etc/portage/package.* from unused entries
 * Forum thread: Find obsolete USE flags in /etc/portage/package.use