app-containers/containers-storage
containers/storage library
USE Flags
btrfs
Global: Enable support for chroots using btrfs snapshots.
+device-mapper
* This flag is undocumented *
test
Global: Workaround to pull in packages needed to run with FEATURES=test. Portage-2.1.2 handles this internally, so don't set it in make.conf/package.use anymore