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IIRC historicall there were some amazing promotions to Brevet Rank and then return to Substantive Rank especialy during the US Civil War
I know nothing about US military but the British army award of a brevet conferred either major, lt-colonel or colonel, and had no effect regimentally but great effect in mixed company.
As an example, Captain [substantive] Stanway, an ex -ranker, was breveted first to major then lt-colonel during the Great War. After the war he served as a captain until retirement but commanded the mixed force which put down the Connaught Rangers mutiny in India. This command would have been effective even if there had been a substantive major in the ad-hoc formation.
He would also have presided over any courts-martial unless a lt-colonel had been among the assembled court.
Not sure about the present day, the rules change quicker than my socks.