A Domain's AUTH-ID
Most types of domains have an AUTH-ID code.
The AUTH-ID code usually is a 6 to 16 character code assigned by the Registrar at the time the name was created.
Its purpose is to aid identification of the domain owner, so proper authority can be established (it is the "password" of the domain).
The AUTH-ID is used and is necessary for doing transfers between registrars.
If you want to transfer a domain to Joker.com, you have to request this AUTH-ID for any domain to transfer from your current registrar or ISP.
If you want to transfer a domain from Joker.com to another registrar, you may request this AUTH-ID in 'My Domains'.