Sector admiral is a fictitious military rank which has appeared in numerous science fiction sources both live action production and literature. The rank of sector admiral is typically considered senior to a fleet admiral and is bestowed upon an admiral who commands all planetary installations and starships operating in a sector of space.
For example, in the Traveller role playing game, sector admirals typically command major naval Depots or the forces assigned to either a sector or domain.
References
Sector admiral Wikipedia(Text) CC BY-SA