as new types were constructed, the Southern Region adopted a more complicated system for their electric multiple units, with the second and third digits indicating in more detail the type of unit. Second digits were allocated as follows: Third digits were allocated as follows: Of course, many exceptions arose over time. One major change was to change the classification of unpowered trailer units from 49x numbers to 4x8 numbers (which involved reclassifying Class 491 to Class 438). When Southern Region unit numbers were changed to fit with the TOPS classification system, former 4x0 classes were all reclassified to 4x2. This was necessary because Southern Region units only displayed the last four digits of their six-digit TOPS number, and it was decided that no painted unit number should commence with a '0'. It is worth noting that despite only showing the last four digits, the actual number of the unit was still the six-digit TOPS number. This often causes confusion both to enthusiasts and those outside the field alike. 2011 numbering and classification alterations and extensions - TOPS In September 2011, the UK Rail Safety and Standards Board issued Railway Group Standard GM/RT2453, which made some alterations to the TOPS classes allocated to various types of locomotive and multiple unit - primarily extending the number ranges for certain types of locomotives and multiple units where the previous allocation ranges were becoming exhausted. The standard also provided for the allocation of European Vehicle Numbers to most mainline rail vehicles in Britain, as part of the implementation of EC Directive 2008/57/EC (the Railway Interoperability Directive). Railway Group Standard GM/RT2453 was replaced in December 2017 by Rail Industry Standard RIS-2453-RST, which introduced the requirement for all new rail vehicles receiving authorisation to enter service in Britain after 1 January 2018 to display their full 12-digit European Vehicle Number regardless of whether or not the vehicle is intended for use only on domestic services. Following the introduction of Standard RIS-2453-RST the allocation ranges have been as follows: Locomotives *A Electric shunting locomotives do not have their own range and may be allocated from the diesel shunting locomotive or the electric locomotive ranges. *B Although InterCity 125 power cars are often considered to be Class 43 diesel locomotives, they are treated as diesel multiple units for the purposes of vehicle numbering. Multiple units and fixed formation sets *A A multi-mode unit is one with two or