/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