cups - Add support for CUPS (Common Unix Printing System)
debug - Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces
doc - Add extra documentation (API, Javadoc, etc). It is recommended to enable per package instead of globally
ffmpeg - Enable ffmpeg/libav-based audio/video codec support
gstreamer - Add support for media-libs/gstreamer (Streaming media)
icu - Enable ICU (Internationalization Components for Unicode) support, using dev-libs/icu