Basis

Summary
Basis is the largest moon orbiting the planet Threshold in the Soell system. The diameter of this moon is 23,848km. Installation 04 was positioned between Basis and Threshold. After Installation 04's destruction, the Covenant Heretics built a makeshift camp on Basis, amid the wreckage from the Alpha Halo. Basis was the location of the Halo 2 multiplayer level Burial Mounds.

In the Halo 2 levels Sacred Icon and Quarantine Zone, you can see Threshold with Basis in the background in the sky. This is most likely an error due to time constraints or they are meant to be similar planets/satellites.