In the Marine Corps, MOS stands for Military Occupational Specialty. All Marine officers and enlisted personnel are assigned a four-digit code that categorizes their career field and specialty.
The first two digits denote the Marine's occupational field, and the second two digits are the job code, describing the Marine's specific responsibilities within his occupational field. For example, an MOS of 0300 is assigned to basic enlisted infantrymen, and 0302 is given to infantry officers. Scout snipers are assigned 0317 and machine gunners are 0331. All prospective recruits must take the Armed Services Vocational Aptitude Battery, which influences which MOS they may volunteer for or be assigned.