Real ID, Battle Tag, whichever you want to call it.
Edit: It was confirmed, there will be 'Real' Cross-Realm Raiding in 5.2. I would have posted in that guy's other post, but I don't like him cause he can't read too well.
@CM_Crithto @Teukli Yes, they should all be crossrealm. The normal and heroic 5.2 raid will not be crossrealm, at least in 5.2.