The rank above Captain, usually assigned to a commander of an 'action group', flotilla, or port. He coordinates actions between several vessels at a tactical level. (Strategy is left up to the Admiralty) For the US Navy, "commodore" usually only gets put into place at wartime, and often redone in peace-time as admiralty. Carter's infamous 'ass admiral, ass half' is the most famous example of 'retiring the rank', but it's actually been done quite often.