From Halopedia, the Halo wiki
The CAA Marshals are a law enforcement agency under the Colonial Administration Authority. Marshals hold jurisdiction on all Unified Earth Government-controlled colonies except for Earth, in which they serve as frontier sheriffs - particularly prevalent in the Outer Colonies. Depending on the size of a given colony, a single Marshal may have responsibility for enforcing UEG law across an entire world or even (in rare cases) a whole solar system.
- Halo Encyclopedia (2022 edition), page 54-55