ratings

United States Navy ratings are general enlisted occupations used by the U.S. Navy since the 18th century, which denote the specific skills and abilities of the sailor. Each naval rating has its own specialty badge, which is worn on the left sleeve of dress uniforms of enlisted personnel. U.S. naval ratings are the equivalent of military occupational specialty codes (MOS codes) used by the United States Army and the United States Marine Corps, the ratings system used by the United States Coast Guard, and Air Force Specialty Codes (AFSC) used by the United States Air Force and United States Space Force.
Ratings should not be confused with rates, which are used to identify personnel of specific a rating and pay grade. For example, if a sailor has the pay-grade of E-5 (petty officer second class) and the rating of boatswain's mate, then combining the two—boatswain's mate second class (BM2)—defines both pay grade and rating in formal address or epistolary salutation. Thus, boatswain's mate second class (BM2) would be that sailor's rate.
Sailors from pay-grades E-1 to E-3 that have no rates, are considered to be in apprenticeships or training for a rating, thus the slang term "undes" (Pronounced UN-DEZ) (un-designated) when referring to them as a group. A Sailor actively working toward a specific rating is referred to as "striking for a rating" and is called a "striker". E-1 to E-3 are divided into five general occupational fields (airman, constructionman, fireman, hospitalman, or seaman) based on their rate. For example, an AD (Aviation Machinist's Mate) E-3 would be referred to as an Airman, an E-2 as an Airman Apprentice, and E-1 as an Airman Recruit. The paper designation for these is ADAN, ADAA, and ADAR respectively, SN, SA, and SR for sea-going rates, FN, FA, FR for engineering and damage control rates, CN, CA, CR for Seabee, naval construction units, and HN, HA, and HR for Corpsman.
Naval Officers: Although naval officers do specialize in various fields their occupations are classified according to designators for both officers of the line (i.e., line officers) and those of the professional staff corps.

View More On Wikipedia.org