Random Dungeon battlegroup

[forever]

I haven't played since the 'merge' took place but can someone explain how it works? I've read that it's all of the battlegroups are one massive pool now, except for rated battlegrounds and arenas, which are restricted to battlegroup still. Someone else said that it's limited to the data centers (3-4 Battlegroups).

So which is it? All BG's or just DC's (3-4 BG's)?