Ever wonder why the Army, Air Force, and Marines seem to have hundreds of enlisted jobs, while the Navy only has a few (comparatively speaking) ratings? The answer is the Navy Enlisted Classification (NEC) system.
The NEC system supplements the enlisted rating structure in identifying personnel on active or inactive duty and billets in manpower authorizations. NEC codes identify a non-rating wide skill, knowledge, aptitude, or qualification that must be documented to identify both people and billets for management purposes.
It helps to think of an NEC as an "advanced specialty" within a job. The other services also use the "advanced specialty within a job" system, in one way or another, but not to the extent that the Navy uses their NEC system. For example, in the Army, "Operating Room Specialist", and "Radiology Specialist" are two separate jobs (MOS 68D and 68P, respectively). The same is true of the Air Force (AFSCs 4N1X1 and 4R0X1 ). In the Navy, an operating room specialist and a radiology specialist hold the same rating (job) -- that of HM (Hospitalman).
So, how does the Navy know which HMs to assign to Navy operating rooms and which HMs to assign to X-ray sections of the hospital? They are assigned an NEC to designate their "advanced specialty." If an HM receives advanced training as a surgical technologist," he/she is then awarded the NEC of HM-8483, and can thereafter be assigned duty assisting Navy surgeons. If a sailor with an HM rating receives advanced training as an x-ray technician, he/she would be awarded the NEC of HM-8451, or HM-8452, and then be assigned to work with Navy radiologists.
Below are the current Navy Enlisted Classification codes, organized by basic communities: