Dormammu I've come to ba... ask questions.

So, my first question was something else but now I'm curious: When it says "claimed by GM" does that mean it's forbidden or is it gonna be used by GMs? Now that I typed out the question I feel like this question inevitably spoils but my curiosity is forbidding me from not asking anyways. ;c

But WAY more importantly: HOW exactly is this AU? I did read OP, of course, and my assumption is: Pre-Naruto-show lore remains but from there on it's our thing? 'cause even though I'm not an expert or anything on the show/manga, as far as I know the lore is pretty fundamentally tied to characters and certain events that kinda enable the whole world and mythos. So I just would like some clarification on that, if possible.