Hull classification symbol
From Halopedia, the Halo wiki
A hull classification symbol, also known as a hull code or a registry, is assigned to UNSC starships of the UNSC Navy. Different types and sizes of ship have different codes. The hull code is composed of a symbol and a hull number. The hull numbers are usually assigned to each ship consecutively, though number sequences may be skipped for a variety of reasons (planned ships that were never commissioned or bureaucratic maneuvering, for example) and older numbers may be reused for new ships in order to "inherit" the identity of a particular storied vessel.
List of known symbols
The symbol is based on the hull classification symbol system used by the United States Navy in the 20th and 21st centuries with a few differences.