UNSC rank structure
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The rank structure of the United Nations Space Command's mainline branches of service (the Navy, the Marine Corps, the Army, and the Air Force) is largely derived from the system employed by the 21st century United States military.
The UNSC's rank structure is similar to that used by 21st century militaries, primarily the United States military. Unlike contemporary military forces, however, the UNSC largely employs two main rank schemes throughout its organization; NAVCOM rankings for organizations within UNSC Naval Command (namely, the UNSC Navy and Office of Naval Intelligence) and UNICOM rankings for Unified Ground Command subordinates (UNSC Army, Air Force, Marine Corps). UNSC ranks can be broadly categorized into several groupings;
UNSC Naval Command uses rank titles which are different to the other branches. Navy ranks and terminology have a long history which dates back to eras when ships sailed on the oceans of Earth rather than in space. These ranks are used by the UNSC Navy and ONI.
UNSC ground forces rely on little administrative overhead and bureaucracy thanks to artificial intelligences and automated services, which handle the majority of routine paperwork and coordination. The UNSC's standards of training and high "tooth to tail" ratio of fighting troops to support personnel also allow senior enlisted and junior officers to be given much more responsibility and command authority than what was historically possible. Some UNSC Army and Air Force personnel additionally make use of ranks and/ or roles that do not fit into the UNICOM system of organisation.[unicomnote 1]
Unlike NAVCOM and UNICOM, the Spartan Operations branch does not employ traditional ranks. Instead, the Spartan branch works to take the informal leadership traditions of the SPARTAN-IIs and adapt them into a structure than emphasizes independence and unity of purpose over rigid adherence to the chain of command. Nevertheless, SPARTAN-IV personnel are given additional responsibilities and authority as they advance in their careers. All Spartans receive a direct commission as an officer of the UNSC, though this authority does not typically extend out of their chain of command.
While aboard naval vessels, Spartans report directly to the ship's captain in all matters relating to the safety and security on-board the ship - superseding all other responsibilities and orders. In all other cases, the Spartan Commander has operational control of Spartan personnel, and Spartans cannot give or be given orders to/from Navy or ONI personnel. Spartans are, however, encouraged to accommodate reasonable requests when they do not violate existing orders or the Spartan Code.