Each Segmentum has its own Imperial Navy and Merchant Fleets, as well as a Segmentum Fortress or Segmentum Naval Base that forms the base of fleet operations within the Segmentum. The Segmentum fleet commanders are known as the Lord High Admirals. The Segmentum Fortress is controlled directly by a high-ranking official of the Administratum known as the Master of the Segmentum.
The Segmentae Majoris and their fortresses are:
- White Dwarf 139 (UK), Interstellar Travel by Jervis Johnson, Andy Jones, Simon Forrest & Rick Priestley
- White Dwarf 140 (UK), Interstellar Travel by Jervis Johnson, Andy Jones, Simon Forrest & Rick Priestley