The copy brings every setting of the source phone: the agent switches, the timeouts, the codecs, the voicemail options. Only what is asked for here is written, and the servers involved are marked so the dialplan is rebuilt.
Two phones cannot share a box: they would overwrite messages.
It cannot be a phone login or a phone alias that already exists.
What the phone uses to register against Asterisk.
Optional. Empty keeps the source value out of the way.