A United States military occupation code, or a military occupational specialty code (MOS code), is a nine-character code used in the United States Army and United States Marines to identify a specific job. In the United States Air Force, a system of Air Force Specialty Codes(AFSC) is used. In the United States Navy, a system of naval ratings and designators are used along with the Navy Enlisted Classification (NEC) system.
Since an individual can obtain multiple job specialties, a duty military occupational specialty (DMOS) is used to identify what their primary job function is at any given time. An individual must complete and pass all required training for their military occupational specialty qualification (MOSQ).
Originally, the MOS system had three to five digits. The first four-digit code number indicated the soldier's job; the first two digits were the field code, the third digit was the sub-specialty and the fourth code number (separated by a period) was the job title. A fifth code digit was for the soldier's special qualification identifier (SQI), which indicated what specialized training the soldier had. If the soldier did not have an SQI, the digit was listed as "0" or was omitted.
One-one is the field code for infantry, 1.1 is the sub-specialty of light weapons, and seven is the SQI for airborne training. Therefore, 111.10 is the MOS for an infantryman and 111.17 is for an airborne-qualified paratrooper. Nine-one was the old field code for the medical field, 912.0 is the MOS for medical NCO and 912.00 is a generalist medical NCO with no SQI.
https://en.m.wikipedia.org/wiki/United_States_military_occupation_code