In the Army & Marines, an enlisted job is called an "MOS" (Military Occupation Specialty). In the Navy & Coast Guard, an enlisted job is called a "Rating." In the Air Force, and enlisted job is known as an "AFSC" (Air Force Specialty Code.
The 1st number in the AFSC is the career group. There are 9 Air Force Career Groups: 1-Operations; 2-Maintenance/Logistics; 3-Support; 4- Medical/Dental; 5-Legal/Chaplain; 6-Finance/Contracting; 7-Special Investigations; 8- Special Duty Assignments; 9-Special Reporting Identifiers.
The 2nd digit (letter) identifies the career field. The 3rd digit (numeral) indicates the career field subdivision (ie, job functional area). The 4th number in the AFSC indicates a person's skill-level. For example, someone with the AFSC "1A051" has a five-skill level. An individual receives the "1" (helper) skill-level when they enter technical school for the AFSC. Upon graduation from technical school, they receive the "3" (apprentice) skill level. Individuals are normally awarded the "5" (journeyman) skill level after a period of on-the-job training (OJT) and correspondence courses (Called "CDCs"). Depending on the job, this process can last anywhere between 12 and 18 months. Upon promotion to Staff Sergeant, individuals enter training for the "7" (craftsman) Skill Level. "7" level training consists of more CDCs, more OJT, and (for some jobs) a 7-level technical school. Upon promotion to E-8, the person receives a "9" (superintendent) skill level.
The final digit (numeral) indicates further job division within the same functional area. Specific skills (such as type of aircraft) are designated by suffixes, such as "A" or "B."