Real refers to reality changes, usually in the sense of "reality adjusts to reflect the change". Such as everyone thinking a certain character has always been a girl.
I agree that the issue with the "be" tag is debatable. I've debated about adding a "big breasts" tag to compensate, but that seems unfair to the other tags.
So THAT'S what that means. Actually, that's a very, very useful tag, but I doubt many people actually know what is intended... my best guess was physical laws changing. At the very least, the description should be more clear, but I would suggest changing the entire tag to "retro - retroactive change". It's concise and (I think) pretty obvious, at least as one-word descriptions go.
BE seems simple enough to me, but I always considered it the fetish rather than the act. If it features growing breasts or enough discussion of growing breasts it fits the fetish and is tagged. Sure there's a grey area in the latter one, but not that much. Now on the other hand a big breasts tag sounds like it's asking for trouble. Should DDs, head-sized breasts, lap-filling breasts and beyond really be under the same tag? If different categories are split up how do you describe them accurately enough to prevent confusion in only two or three words? (I may have accidentally answered my own question)
(A segment of this post has been removed because it discussed prohibited topics. —moderator)