A rating code was the Romulan equivalent of a personal ID card or credentials.

When it was doubted that Admiral Alidar Jarok was just the low-level logistics clerk Setal, the Romulan offered to show the crew of the USS Enterprise-D his rating code to prove that he was Jetal. Worf, however, claimed that these codes could easily be forged by a spy. (TNG: "The Defector")