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 (MOSs).
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.
The Marine Corps used to have two basic types of MOSs -- Primary MOSs (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" MOSs and replaced them with NMOS, FMOS, EMOS, and AMOS:
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 a 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.