Mostly it's just the "System" subs, but it's always been vaguely defined.  Seems to be if you created the sub from scratch, you can do what you like. You'll be attacked, but no one can force you to do shit (see @HenryCorp and his subs). In the main subs that are modded by @system you can say anything that complies with US law, with the exception of doxxing site users, apparently.  At least this is way Voat has functioned up til now.