From Halopedia, the Halo wiki
A colony starter unit is a structure capable of being deployed by UNSC colonization teams during the buildup of initial settlements on a planet's surface. Constructed from polycrete, and powered by a small hydrogen reactor, they were extremely useful during the UNSC's campaign to put down the Insurrection, with modified versions of these forming the basis for the Firebase.
The military variant of the colony starter unit starts as a Firebase featuring 3 sockets for buildings and none for turrets. After upgrading once the base has 5 sockets and 4 turret sockets. After the final upgrade it has 7 building sockets but still only has 4 turret sockets.
List of appearances