On one end of vehicles (the non-cab end of a power car - unless a bubble car!), on the left hand side is normally found data particular to that vehicle.
The data carried varied extensively and would grow over the years. Some of the details may be the class, dimensions, diagram, weight, vehicle type, seating, new date, lifted date, paint and shopping proposal (SP) date.
The example image is from a brand new Derby Lightweight vehicle, and all this is hand painted on by a signwriter. From the top: the weight; dimensions; the vehicle type, BTO being a Brake Third Open; Shopping Proposal date (when it is scheduled to return to works for maintenance / overhaul; New from 2001 (see next paragraph) and the new date, the finally the painting date with a reference to synthetic paint being used.
The depot / works that performed any work was reported by a numerical code, different to the usual recognisable depot codes. Below are listed some from the LMR, and the following pages contain those from the ER, NER and ScR. Few of these places would deal with DMUs but are included for completeness. So the code in the image means that the vehicle was built at Derby Works (code 2001) rather than being built in the year 2001!
2001 Derby Works
2110 Derby Etches Park