Classification
The early AC overhead electric multiple unit types were referred to by a two-letter code. This was adapted for the DC third-rail system that was adopted as standard by the Southern Railway, with unit type being given a three-letter code (sometimes two letters) prefixed by the number of carriages within each unit, e.g. 4SUB for a 4-car suburban set.
The Southern Region of British Rail perpetuated this method of classification, and the same principles were also adopted for diesel-electric multiple unit stock, but with single-letter codes. The last type to be officially allocated a designation in this series was the PEP stock of the early 1970s. However, some types built since then have been given semi-official designations in this style.
NB: This was different from the system used by British Rail (adopted from the LNER) to indicate the type of non-powered coach — see British Rail coach designations.
Read more about this topic: Southern Railway Multiple Unit Numbering And Classification