I've heard that if an army/marine/air force captain (an O-3, equivalent to a navy/Starfleet lieutenant) is on a ship, he is called "major" rather than captain, since a ship can have only one captain. What do you call a naval captain (O-6), however, who is on a ship not his own (either in real naval parlance or in the Starfleet universe)? He can't be nominally "promoted" like a marine captain to a marine major, since the next highest rank -- admiral (or commodore) then outranks the ship's captain. Is he nominally "demoted," i.e., do people call him commander? Is the one-captain rule waived for other naval personnel, as opposed to the lower army/marine/air force "captains"?