Well the "Mü" part is standard if you live here. Someone from Hamburg would be "HH" while a Berliner would be "B". So basically, it depends where you live in Germany. Munich is "M", Nürnberg is "N" etc.

The second letters in this case, BM, can be chosen by the owner of the car. For example, the license plate of my dad's 500SL is EC 888. My dad's name is Erich and he thought it would be cool if the license plate basically said "Erich+Christian". The three 8's are some good luck number in Chinese I believe. My dad was fascinated with Chinese culture and their superstitions.