I don't know if they still have them, but the brevet promotion was a temporary promotion without pay in the 19th century armies. It allowed commanders to fill the ranks of their fallen officers in the field without committing the army to confirming the promotion under less stressful conditions. Sometimes the brevet rank persisted for a long time, and people resigned or were demobbed at a brevet rank. Usually, I think, they were entitled to refer to themselves as being that rank, i.e. a captain who was commissioned to brevet major was entitled to continue to call himself "Major" provided the brevet was never withdrawn.

HTH