I prefer to apply the Royal Navy explanation, from what I know of Napoleanic wars and prior a lieutenant could command a sloop of war which could be a vessel from a sloop on up to a brig or corvette, and he would be elevated to Commander, or "Master and Commander" of that vessel, though he would be referred to as Captain by courtesy and commander was not an official rank. He could not command a a "Post" ship however, such as a Frigate or Ship of the Line, those required a Captain's rank.
-
In Elite, we command the sloops of war, so we are Commanders regardless of our actual rank. The "Post" ships (Farragut, Interdictor, etc) these are for the Captains to command.