For those unfamiliar, a brevet rank is basically a temporary rank, generally issued during times of war when there are shortages of personnel in key positions. It’s used to recognize an individual and give them the authority of their brevet rank even if they don’t meet the requirements for said rank, generally time in service and grade. In this example, one usually has to be in the British Army for 12 years before being promoted from Corporal to Sergeant. Akko is only a Brevet Sergeant because the SMS Witches are desperate for capable leadership and because she impressed the cadre at the SMS/SAS course. If she messes up at her receiving unit, she can be demoted very easily, but they’ll give her the benefit of the doubt at first...after all, they know the standards by which those cadre measure their students!