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
sasl - Add support for the Simple Authentication and Security Layer
seccomp - Enable seccomp (secure computing mode) to perform system call filtering at runtime to increase security of programs
selinux - !!internal use only!! Security Enhanced Linux support, this must be set by the selinux profile or breakage will occur
slabs-reassign - Allow manual reassignment of memory slabs at the cost of slab optimizations.
ssl - Add support for SSL/TLS connections (Secure Socket Layer / Transport Layer Security)
test - Enable dependencies and/or preparations necessary to run tests (usually controlled by FEATURES=test but can be toggled independently)