From Lexicanum

A Segmentum Fortress is an orbital base for fleet operations throughout its associated Segmentum. Each of the five Segmentae, the Imperium's primary administrative/fleet zones, has a Segmentum Fortress or fleet base.

These bases are huge ports with docks, shipyards and repair facilities. In reality however, relatively few of the fleet ships ever travel to their theoretical base. The main function of a fleet base is fleet administration.

A Segmentum Fortress is controlled directly by an Administratum Master.

Locations of the five Segmentum fleet bases:

• Bakka - Segmentum Tempestus
• Cypra Mundi - Segmentum Obscurus
• Hydraphur - Segmentum Pacificus
• Mars - Segmentum Solar
• Kar Duniash - Ultima Segmentum

Sources

• White Dwarf issues 139 and 140, Interstellar Travel, by Jervis Johnson, Andy Jones, Simon Forrest & Rick Priestley