Transponder codes are assigned in the UK by a section of the CAA in the Directorate of Airspace Policy (DAP). If they are not ORCAM codes, eg are assigned to individual airfields or ATSU's, DAP have to ensure they are not duplicated by with the assignment to neighbouring units in order to avoid the two units assigning the same code to different aircraft in the same area, for instance, the Farnborough 04XX allocation is also assigned to Lakenheath and Birmingham, all three being deemed to be far enough apart to avoid confusion.