Brand new player here, a bit confused...

So data centers, are group finders locked to the realm you pick, or the region your realm is part of? Let's say I pick "Aether" and play on "Siren" Can I group with people from the same region like "Cactuar" Or just people from "Siren"?

Everyone keeps telling me to go "Balmung" But I hear it's dead, if that's the case what's the highest populated realm I should go to?