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The Navy Enlisted Classification (NEC) system supplements the rating designators for enlisted members of the United States Navy. A naval rating and NEC designator are similar to the Military Occupational Specialty (MOS) designators used in the U.S. Army and U.S. Marine Corps and the Air Force Specialty Code (AFSC) used in the U.S. Air Force.
The U.S. Navy has several ratings or job specialties for its enlisted members. An enlisted member is known by the enlisted rating, for example, a Machinist's Mate (or MM or by the enlisted rate, for example Petty Officer First Class (or PO1). Often Navy enlisted members are addressed by a combination of rating and rate. In this example, this machinist's mate petty officer first class may be addressed as Machinist's Mate 1st Class (or MM1).
However, the NEC designator is a four-digit code that identifies skills and abilities beyond the standard (or outward) rating designator. According to the Military Personnel Manual (MILSPERMAN) 1221-010, the NEC designator facilitates personnel planning, procurement, and selection for training; development of training requirements; promotion, distribution, assignment and the orderly call to active duty of inactive duty personnel in times of national emergency or mobilization.