The United States Navy

Enlisted members of the Navy have pay grades from E-1 to E-9, with E-9 being the highest. All enlisted sailors with paygrades of E-4 and higher are considered Petty Officers while those at E-7 and higher are further named Chief Petty Officers. Those who demonstrate superior performance are given an increase in paygrade; the official Navy term is to be advanced. Two notable advancements are from Seaman to Petty Officer Third Class (E-3 to E-4) and from Petty Officer First Class to Chief Petty Officer (E-6 to E-7). Advancement to Chief Petty Officer is especially significant and is marked by a special induction ceremony.
Enlisted members are said to be "rated", meaning that they possess a rating, or occupational specialty. Members of grades E-1 to E-3 can become "strikers", meaning they have rating designations like Petty Officer (example: a BM3 is a Petty Officer Third Class rated as a Boatswain's Mate; BMSN is a Seaman designated as a Boatswain's Mate striker), but the striker is doing on the job training to become a rated petty officer rather than attending a school to become rated. There are more than 50 ratings covering a broad range of skills and subspecialties. However most sailors in today's navy with grades E-1 through E-6 obtain their rating through its respective "A" school. An "A" school is a rating specific school where sailors are trained as experts in their field. Upon completion of their training they are considered "Rated", regardless of their pay-grade.
For example, SA SMITH, MARY, would be considered a Seaman Apprentice. Prior to her rank of SA a rating would be placed. Therefore, her entire title would be ITSA SMITH, MARY. IT indicating that she is an Information Systems Technician. As for ENFN THOMPSON, JOHN. EN specifying that he is an Engineman and FN as Fireman.