In case you're still wondering what it is and what it's trying to accomplish, take a look at this page:
https://wiki.gentoo.org/wiki/Categories_acct-group_and_acct-user
As for funtoo policies, afaik this practice has not been adopted here (yet?), so every ebuild does its own user-group management. I think a good example of this is the www-servers/lighttpd ebuild.