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The keystone v3 API contains the following statement in the Users section

Either globally or domain unique username, depending on owning domain.

Can someone explain what this means please.

More specifically, this states that a username is either globally unique 
across all domains, or is locally defined in a domain.

First question. How can anyone tell the difference between a globally 
unique username and a domain specific username?

Second, who or what is the owning domain for a globally unique username?

Finally why should the owning domain determine whether the username is 
globally unique or not? What if owning domain 1 determines that 
username1 is globally unique and owning domain 2 determines that 
username1 is locally unique to itself?

thanks

David