Sector

A sector is a region of space as identified by the United Nations Space Command, 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.

There appear to be two systems for naming sectors, one for inhabited sectors, one for uninhabited sectors. The UNSC Fleet Command designates sectors as "FLEETCOM Sector (number)" to denote the division of command the sector's inhabitants fall under. Other sectors are simply designated "Sector (letter)-(three numbers)" or informally by name.

FLEETCOM sectors

 * FLEETCOM Sector One
 * FLEETCOM Sector Three
 * FLEETCOM Sector Four

Other sectors

 * Eridanus sector
 * Sector B-042
 * Sector K-009
 * Sub-Sector 35