I don't think there's any official criteria but all the guys who've fought for it, have all been known for being in a lot of "wars". It's not about who is the most dominant or best, but guys who have big name value while also being known as disciples of the Just Bleed God. That's why I think Ilia shouldn't fight for it, to his credit, he's been so dominant that he's never been in a real "war" fight in the UFC