I think they are just per-name (DBO)

by Blackt1g3r @, Login is from an untrusted domain in MN, Wednesday, August 30, 2017, 09:33 (2430 days ago) @ INSANEdrive

I'm pretty sure they choose a random number to add to your "Battle Tag". Previously they used a system (called RealID) where you used people's email addresses, but they got rid of that in favor of this system so your email address is not exposed.

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