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
go - Enable building dev-lang/go code (FvwmPrompt)
nls - Add Native Language Support (using gettext - GNU locale utilities)
perl - Add optional support/bindings for the Perl language
vanilla - Do not add extra patches which change default behaviour; DO NOT USE THIS ON A GLOBAL SCALE as the severity of the meaning changes drastically