I may be wrong, (been 17 years since I was in service) but brevet was an "acting" rank, usually without the pay increase, ie. a Colonel, brevetted a Brig. General, to command a brigade instead of a regiment. The "brevet" rank was/is not always used, as one of my ancestors Moses Jackson, commanded the 33rd Miss. Inf. Regt as a Capt, after Col. Drake was killed.