In the JETDS system, complete equipment sets or systems are designated with a sequence of letters and digits prefixed by AN/, then three letters, a hyphen, a number, and (occasionally) some optional letters (AN/AAA-nnn suffixed by (Vn) {hardware/software version} or (T) {training equipment} . The three letters tell where the equipment is used, what it does and its purpose. For example, the AN/PRC-77 is a Portable Radio used for two way Communications. The model numbers for any given type of equipment are assigned sequentially, thus higher numbers indicate systems that are more modern.