Sergeant major

Sergeant Major is an E-9 rank of the UNSC Marine Corps and UNSC Army. It is a staff non-commissioned officer rank. It is equal in pay grade to Master Gunnery Sergeant in the Marines or Command Sergeant Major in the Army, although the two have different responsibilities. Sergeant Major is equal to the rank of Master Chief Petty Officer in the UNSC Navy. A Marine Sgt Maj is similar to a Command Chief Master Sergeant in the UNSC Air Force and Command Sergeant Major in the Army. An Army SGM is similar to a Chief Master Sergeant in the Air Force or a Master Gunnery Sergeant in the Marines.

Sergeant Major and Master Gunnery Sergeant
In the United States Marine Corps, eligible Gunnery Sergeants indicate whether they wish to be considered for promotion to Master Sergeant or First Sergeant, and thus whether they enter the promotional track for Master Gunnery Sergeant or Sergeant Major. Once promoted, the promotional track is permanent; lateral movement between the two programs is not possible.

One of the major differences between the two ranks is that Sergeants Major have a general command status, while a Master Gunnery Sergeant's role involves providing technical military leadership.

Whether this carries over into the UNSC Marine Corps is unconfirmed, but is likely.

Marine Corps
Although technically the same rank as a Master Gunnery Sergeant, Sergeants Major are given more authority. They serve as the senior NCO for battalion level units and above, advising and assisting the battalion (or above) commander directly, and are responsible for all enlisted marines in the unit.

Army
The Army Sergeant Major rank is similar to a Marine Corps Master Gunnery Sergeant. SGMs typically hold a staff position, assisting higher ranking staff officers in their duties.