22nd Tactical
From Halopedia, the Halo wiki
The 22nd Tactical was a United Nations Space Command Marine unit stationed aboard the UNSC Pillar of Autumn. It was to rendezvous with the Fourteenth Platoon at Bulkhead Charlie 14 aboard the Pillar of Autumn during the Covenant boarding action.
The 22nd Tactical was most likely Major Silva's ODST battalion. In this case the unit's full name would be the 22nd Tactical ODST Battalion. This would mean that Bulkhead Charlie 14 was in or near the Autumn's Engineering section, since the ODSTs were tasked with defending the ship's reactors.
