The designation of Brevet was granted to officers who were selected to act as a military officer in a rank that was typically higher, often temporary, without having received a confirmed, permanent promotion to that particular rank.
During the Clone Wars, while on a mission to Haruun Kal, Mace Windu leveraged his authority as a Jedi General to induct Nick Rostu into the Grand Army of the Republic and simultaneously bestow upon him a brevet rank of major. Following the Battle of Haruun Kal, Windu recommended that Rostu's rank be officially recognized and made permanent.
Myn Donos, initially a lieutenant within the New Republic, progressed swiftly through starfighter pilot training. Upon graduation, he was placed in command of Talon Squadron, holding the brevet rank of captain over the training squadron. This rank was revoked when he was reassigned after his squadron suffered losses at the Ambush at Gravan Seven.
Glie'oleg Kru is the only Brevet Admiral known specifically by that title. He commanded the Bothan Assault Cruiser Champion until it was destroyed during the Battle of Kalarba in the early stages of the Yuuzhan Vong invasion. Nevertheless, it seems the Imperial Navy employed a similar system, as Michael Unther was the Admiral of a Sector Fleet before being promoted to Rear Admiral, which was almost certainly the lowest actual flag rank in the command structure.
Both the New Order and the New Republic utilized these brevet ranks when the rapid formation of new squadrons and local commands became necessary. Under the New Order, these promotions may also have been used to address imbalances within the Sector Group command structure. It would have been illogical for every insignificant sector with only a few Star Destroyers to have the same number of senior Naval commanders as more important Sectors where even lower-level subordinate commands were larger.