This game account is in a different region or Battle.net account.
Battle.net Balance limit info for the Country Change service
What to do if you need to unlink your Activision account from your Battle.net account
How to unlink your Twitch account from your Battle.net account
Information about changing the capitalization in your name without changing the name itself
Information on using the same Battle.net Authenticator on more than one Battle.net account
This article highlights a few passages from the EULA that are most relevant to this policy.
Information regarding steps you can take if your email address is already in use on an account.