Used to identify the primary skills and knowledge of a Marine. Only enlisted Marines, warrant officers, chief warrant officers, and limited duty officers are promoted in their primary MOS. Changes to an
Active Component Marine's PMOS without approval from CMC (MM) and changes to a RC Marine's PMOS without approval from CMC (RA) are not authorized.
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.
When entered as a requirement into the Total Force Structure Management System (TFSMS), a billet
bearing a necessary MOS must identify a single associated PMOS even if several PMOSs are acceptable
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 FMOS, but include exceptions that require a PMOS.
Additional MOS (AMOS)
Any existing PMOS awarded to a Marine who already holds a PMOS. Example: After a lateral move a Marine's previous PMOS becomes an AMOS. Marines are not promoted in an AMOS.