Sergeant major

Sergeant Major is the ninth and highest enlisted rank of the UNSC Marine Corps and is a non-commissioned officer. It is equal in pay grade to a Master Gunnery Sergeant, although the two have different responsibilities. Sergeant Major is equal to the rank of Master Chief Petty Officer in the UNSC Navy.

Sergeant Major vs. 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 Sergeant Majors 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.

Responsibilities
Although technically the same rank as a Master Gunnery Sergeant, Sergeant Majors 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.