From Gentoo Wiki
The FEATURES variable contains a list of Portage features that the user wants enabled on the system, effectively influencing Portage's behavior. It is set by default via /usr/share/portage/config/make.globals, but can be easily updated through /etc/portage/make.conf. Since this is an incremental variable, FEATURES values can be added without directly overriding the ones implemented through the Gentoo profile.
portageq can be used to show the active features:
portageq envvar FEATURES | xargs -n 1
The descriptions of the FEATURES are available in the man page for make.conf:
FEATURES is often prepended to the emerge command for e.g. keeping the build log or triggering tests:
FEATURES="keeptemp test" emerge -1a foobar
- FEATURES in the Gentoo developer manual