(1) The British Admiralty gave them a capital letter of the alphabet- A B C etc, but not necessarily in the order these ships went to sea, but, in the order that the Admiralty became aware of their identity.
(2) The original name of the ship, as a Merchant vessel. eg "Atlantis," had been "Goldenfels."
(3) The actual name that was given to the Raider, usually by the particular Captain who was first in command. Rogge had christened his Raider, "Atlantis."
(4) A number prefaced by Ship, used by SKL in both signals and correspondence.
(5) There was an HSK number, used to identify a Raider undergoing her conversion, and In dockyard hands.