From Halopedia, the Halo wiki
A non-commissioned officer (NCO), sometimes colloquially referred to as a noncom, is a military officer that lacks a formal commission. Within the rank structure of the UNSC Armed Forces, non-commissioned officers are enlisted personnel that fill most intermediary leadership and administrative roles. The UNSC Armed Forces relies heavily on non-commissioned officers, and they share many responsibilities with officers.
Responsibilities and function
The responsibilities and duties of a non-commissioned officer vary by service, occupational specialty, and duty station. An NCO typically serves as the intermediate between enlisted personnel and commissioned officers. The administrative responsibilities of NCOs include managing logistics, serving as drill instructors, enrolling subordinates into training programs, and reviewing leave requests from junior enlisted personnel.
Within the UNSC Navy, non-commissioned officers typically serve as crew supervisors and senior NCOs may have oversight of a ship's enlisted personnel. Master chief petty officers may even hold fleet administration responsibilities or serve as staff for High Command. Within the Unified Ground Command, non-commissioned officers have command of smaller units, such as squads and fireteams. In larger units, NCOs hold command positions over junior enlisted personnel and serve as an assistant commander, such as a platoon sergeant or battalion first sergeant, and may have administrative duties within the unit.
In order to advance to become a commissioned officer, NCOs must graduate from an Officer Candidate School and take an oath of service. A senior NCO may have enough authority to politely dress down a junior officer and even flag officers may tread lightly around senior NCOs.
The following ranks belong to non-commissioned officers: