Under Republic terminology, a Sector was defined as having a maximum of fifty inhabited star systems. Many sectors grew larger than intended, however, due to the rapid galactic expansion and colonization beginning around 5,000 BBY.
Under the Empire, Palpatine replaced senators with Moffs as Sectorial governors. Many Sectors were rearranged, expanded, or collapsed all together as Palpatine saw fit. Years later, at the recommendations of Wilhuff Tarkin, Sectors were further clumped together in Oversectors, ruled by a Grand Moff.Many Sectors owed their name to their primary system which in turn was named after its primary planet. For example, the planet Guritsan belonged to the Guritsan system, and thus, to the Guritsan Sector.