A sector commander was an official who oversaw all Starfleet operations and starships within a particular sector. This was typically accomplished from a starbase.
During the mid-22nd century, Admiral Uttan Narsu was the sector commander of the sector containing Starbase 12. (TNG: "Power Play")
In 2365, Vice Admiral Nakamura was made the sector commander of Sector 23, and his office was at the newly-built Starbase 173. (TNG: "The Measure Of A Man")