There does not seem to be any index of all the keywords used by portage, in particular within ebuilds. As we wonder about the meaning of such or such keyword, we can build up the index below, with links to the most appropriate documentation.
Another partial list can be found here:
Install Functions Reference https://devmanual.gentoo.org/function-reference/install-functions/index....
In alphabetical order:
CHECKREQS_DISK_BUILD: Ensure we have enough disk space to compile. Typical use in .ebuild file: