From Halopedia, the Halo wiki
A battalion consists of four companies, plus a group of personnel assigned as "Battalion Headquarters Section", for 800 men in total. Under the UNICOM rank structure, a battalion is commanded by an O-5 Lieutenant Colonel, with an O-4 Major serving as a battalion's Executive Officer (XO). The battalion will also have a "first sergeant", usually a Gunnery Sergeant, First Sergeant or above. Three battalions comprise a Regiment.
Within a regiment, the battalions are named numerically, beginning with First battalion and ending with Third battalion. The battalion's subordinate companies are named phonetically, "A" (Alpha) through "M" (Mike), skipping "J". First battalion's companies comprise A through D, Second battalion's companies E through H and Third Battalion companies I through M.
In the UNSC Army, Battalions allocate Fire Support Teams as part of the battalion's offensive/ defensive plans. Their loadout is usually tailored to their mission, terrain, and expected enemy composition. In defensive operations they provide overwatching support-by-fire, while on the offense they engage enemy vehicles and heavy infantry.
List of appearances