From Halopedia, the Halo wiki
Captain, abbreviated to CAPT, is a commissioned officer rank of O-6 in the United Nations Space Command Naval Command (NAVCOM) rank structure; including the UNSC Navy and Office of Naval Intelligence. It is preceded by Commander, and subordinate to a Rear Admiral.
Captain of line warship or installation. Captains have also been shown to command entire fleets and battle groups, an example being Captains Andrew Del Rio and Thomas Lasky commanding the Expeditionary Strike Group 1 Fleet and Captain Maribeau Veredi commanding Battle Group 4.
- Halo: Official Spartan Field Manual - PART 8: UNSC ORGANISATION, NAVCOM Ranks, page 140-141