There doesn't have to be registration or a consistent identity with ssh, when it's used as described in TFA. Just pass a different identity file with "-i".
Of course, most uses of ssh would use a preestablished account on a host and that wouldn't vary, but there's nothing in the protocol requiring that.
Of course, most uses of ssh would use a preestablished account on a host and that wouldn't vary, but there's nothing in the protocol requiring that.