 The First Sergeant and Sergeant Major ranks are command-oriented Senior Enlisted Advisors, with Marines of these ranks serving as the senior enlisted Marines in a unit, charged to assist the commanding officer in matters of discipline, administration, and the morale and welfare of the unit.Master Sergeants and Master Gunnery Sergeants provide technical leadership as occupational specialists in their specific MOS.First Sergeants typically serve as the senior enlisted Marine in a company, battery, or other unit at similar echelon, while Sergeants Major serve the same role in battalions, squadrons, or larger units.