We roll snapshots labeled with a specific RC number more as a level (e.g. ALPHAx, BETAx, RCx). We used to do more formal "releases" but then people tended to get stuck on them and think they never needed to update to a -RELEASE.
So these days, we may occasionally roll one out formally, but most of the time the only "releases" are actually the -RELEASE images.
An RC snapshot could technically become -RELEASE if we wanted it to, though the version number would need updated and a new build performed, everything else would be the same.
Typically when it's time for that to happen we shut down access to the snapshots server for a while so people don't grab a -RELEASE image that isn't really the one, true, final -RELEASE if we find things that necessitate rolling a new set of images.