It isn't e "real" title - it's one we bestow on our dogs to indicate their accomplishments. It doesn't come from AKC at all. I've also seen MBISS which I assume means "multiple best in specialty show" wins, though I don't see that used very often.
I think this is incorrect. It means Best In Specialty Show. Best In Show Special may not be correct, as the dog may not be a special yet. An unfinished dog can and does win BOB at specialties.
It is not a title, just how some people list their dog's accomplishments in print. If a dog has gone Best In Show, or Best in Specialty Show, etc. some people want to draw attention to those wins by adding the corresponding letters to the listed name as if they were titles. I suppose you could even stretch that to BISBBE to mean that a dog won the group and BIS in a Bred By Exhibitor competition, which has become popular at some show. It isn't a title, it is just a brag