Railway coaches are classified under an international system developed by the UIC. This UIC classification of railway coaches replaced earlier national classification schemes in many countries, such as Germany. The coach class is expressed as a combination of letters. It is sometimes followed, for example in the Deutsche Bahn AG, by a three-figure class number. In a broader sense the vehicle number displayed on the coach is also part of its classification, because it encodes other technical details such as the top speed or the type of heating system used.
For example the full designation of a passenger coach, including its coach number, looks like this:
73 80 29-94 708-7 Bpmbz 293.6
This article explains how this classification code on the sole bar of each coach may be decoded.