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
l10n_ru - Install some additional russian docs if 'doc' useflag enabled
ogre - Use the ogre render subsystem. (cannot combine with opengl)
opengl - Use the opengl render subsystem. (cannot combine with ogre)