The markings on the door is normally the load class data of the vehicle (max. load, weight of the vehicle,..). There's usually 4 to 5 lines of data, but then a lot of vehicles do not seem to have them at all (al least not on the doors), and they are sometimes in a rectangle (often in black or a very dark colour), sometimes not (data is often in white then).
I might add that the black rectangle is sometimes made up of dotted lines, sometimes of four bars that do not touch each-other and sometimes is a complete rectangle.
On a typical 4-line marking you'll find:
1st line: type of vehicle
2nd line: Net weight (or in some cases Net weight + Max load)
3rd line: Max load
4th line: Load Class
In the case of a Kfz.2 car, you'd find:
Kfz. Kfz.2
Leergewicht 1225kg
Nutzluse 425kg
Vc.Kl. II
The actual load class (II in this example) depends on the vehicle weight and its length. I, Ia, Ib, II, III, IV, A and S are the different classes.