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 the Gentoo profile, 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
- FEATURES in the Gentoo developer manual