Sector 16 was a region of space somewhere in the galaxy's Alpha or Beta Quadrant.
History and specifics
Circa 2267, Sector 16 was the site of routine Starfleet patrol operations along the outer regions of Federation-controlled space. During this period, the USS Bradbury was assigned to a standard patrol mission within the sector.
While conducting a scheduled pass near the Selko asteroid, designated Gamma-4, the Bradbury encountered an unprecedented superluminal energy phenomenon described as a massive "superpulse" of anomalous cosmic ray emissions. Shipboard sensors registered the pulse traveling at approximately warp factor 34, far exceeding known natural phenomena. (ST short story: "Captain's Log: Stardate 4122.1")