The Marine Corps Occupational System uses a four-digit number code constructed on the concept that occupations with similar skill, knowledge, or functional application requirements are grouped into functional areas, known as occupational fields (OccFlds), and skill-knowledge sets, known as Military Occupational Specialties (MOS).
All branches of the U.S. military designated different occupations using alphanumeric codes. The Army's MOS are generally two digits followed by a letter (so for example, Army MOS 38B is a Civil Affairs Specialist, MOS 21B is a Combat Engineer, and so on).
In the Air Force, jobs are designated with Air Force Specialty Codes, which use a combination of numerals and letters, while the Navy uses its own Navy Enlisted Classification system.
In the Marines, MOS are designated with a four-digit numeric code. Here's what those numbers mean.
U.S. Marines Occupational Fields
OccFlds are identified by the first two digits of the four-digit code and a descriptive title. The OccFld is a grouping of related MOSs. The MOS is a four-digit code consisting of the OccFld code completed by two additional digits. It describes a set of related duties and tasks that extend over one or more grades.
So for instance, a Marine job that begins with the numbers 03 is in the infantry career field. MOS 0311 is an infantry rifleman, 0331 is a machine gunner, 0341 is a mortarman, and so on.
Marines Military Occupational Specialties
The Marine Corps used to have two basic types of MOS -- Primary MOS (PMOS), and Category "B" MOS. Category "B" MOSs were sort of a job-within-a job. A Marine who possessed a PMOS and received extra training in a specialized procedure or equipment could then also be awarded the Category "B" MOS that related to that specialized training.
However, in 2007, the Marine Corps eliminated Category "B" MOS and replaced them with Primary, Necessary, Free, Exception and Additional categories:
- Primary MOS (PMOS): Used to identify the primary skills and knowledge of a Marine. In other words, it is the Marine's primary military job.
- Necessary MOS (NMOS): A non-PMOS that has a prerequisite of one or more PMOSs. This MOS identifies a particular skill or training that is in addition to a Marine's PMOS but can only be filled by a Marine with a specific PMOS.
- Free MOS (FMOS): Non-PMOS that can be filled by any Marine regardless of primary MOS. A free MOS requires skill sets unrelated to primary skills.
- Exception MOS (EMOS): Non-PMOS that is generally an FMOS, but include exceptions that require a specific PMOS.
- Additional MOS (AMOS): Any existing PMOS awarded to a Marine who already holds a PMOS. Marines are not promoted in an AMOS.