/etc/portage/profile/use.mask

Masking
This file can be used for system wide masking of USE flags. As an example, if doc support is unwanted generally it can be masked by adding it here like:

Unmasking
Also this file can be used for unmasking of USE flags which were masked by the developers in the Gentoo repository. Those masks can be found either in or in arch specific file
 * /var/db/repos/gentoo/profiles/arch/base/use.mask
 * /var/db/repos/gentoo/profiles/arch//use.mask

For unmasking such masked USE flag needs to be added here with a leading minus sign (-).

Format

 * Comments begin with  (no inline comments).
 * One USE flag value per line.

External resources

 * Profiles use.mask File on devmanual.gentoo.org