The Southern Railway created classification and numbering systems for its large fleet of electric multiple units, perpetuated by the Southern Region of British Rail until the early 1980s, when the impact of TOPS was felt.
The early AC electric multiple units were referred to by a two-letter code.
The Southern Region perpetuated this, and the same principles were adopted for diesel-electric multiple units 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.
This series was abandoned in 1983, when units were renumbered to fit in with the TOPS classification system, which had nominally been in use for a decade.