N.B.: This rank has not yet been mentioned in the Halo Universe.
Sergeant Major of the Army is the highest enlisted rank of the UNSC Army. It is directly above Command Sergeant Major but below Warrant Officer. It is the equivalent to the ranks of Master Chief Petty Officer of the Navy, Sergeant Major of the Marine Corps, and Chief Master Sergeant of the Air Force.
The SMA is the top enlisted soldier in the Army, and is responsible for all enlisted soldiers. He/she represents them at the highest levels of the military and government. The SMA is similar to a First Sergeant or Command Sergeant Major, though SMA fulfills these duties for the entire Army. As such, only one soldier can hold this rank at any given time.