A brevet rank was/is an “entitled” rank, often issued for valor or meritorious service. They were also issued to officers to fill higher ranks until authorizations were available. Thus, a major may hold the brevet rank of colonel, either through honor or regimental necessity, but not receive the pay of that rank. Brevets were common before, during and after the Civil War.
By “entitled” I mean it was more honorary than anything. The officer may wear the insignia of that rank, but the pay and often the responsibility wasn’t there. Because of his brevet of major general (awarded during the Civil War), Custer was often referred to as “General” when his actual service rank was Lieutenant Colonel.
Brevets were common during the Civil War. In fact an officer may hold more than one, based on his service with volunteer and regular Army units. Things could get real confusing real fast.
The big bugaboo was date of rank. A captain would be senior over another captain of the same regiment because his date of rank occurred first. Even promotions issued the same day in the same general order may be ranked upon who’s name was listed ahead of another. Big arguments ensued over this throughout the 19th Century.
I stand corrected...