In Traveller's canon campaign background, the Third Imperium is subdivided into Domains. Each domain encompasses four adjacent sectors, and is generally under the control of a Archduke, the second highest rank in the Imperium.
Emperor Artemsus established six domains in 76. These were to be administered by Archdukes, and were designed to better manage the Pacification Campaigns (76-120). The Domain of Deneb was established in 589 by Empress Jaqueline I.
The domains of the Imperium are Antares , Deneb , Gateway , Ilelish , Sol , Sylea (administered directly by the Emperor), and Vland.