From Halopedia, the Halo wiki
A sector is a region of space as identified by the United Nations Space Command. Sectors are groupings of Inner and Outer Colony systems organized by the most efficient slipstream space routes branching away from Reach in the Epsilon Eridani system. Fleets of the UNSC Navy are traditionally assigned to sectors; though many fleets were relocated to the Epsilon Eridani and Sol systems towards the end of the Human-Covenant War, the UNSC still maintained these divisions.
The UNSC uses at least three distinct naming systems for sectors. The UNSC Fleet Command designates sectors as "FLEETCOM Sector (number)" to denote the division of command the systems within the sector fall under. Other sectors are designated simply by name, or by the format "Sector (letter)-(three numbers)".