It may seem odd that German military vehicles – not tanks but other vehicles – had license plates or number plates as the Brits call them. But they did. One sees them in lots of photographs of German vehicles although as the war goes on one notices the plates are either missing or have been painted over or smeared with oil since the back color of the plates was white and stood out. Tanks had serial numbers but you couldn’t see them. The markings on tanks were normally a large three digit number painted on each side of their turret and often on the back of the turret. This was their radio call sign so their squadron commander could identify and direct specific tanks under his command to do specific things instead of just saying over his radio, “hey you, the tank under the tree…”. Soviet tanks did not have radios so once a battle started they could not be controlled by a superior officer which is why they normally attacked in waves.
One of the reasons to have license plates was probably because this was the German Army and they wanted order. Secondly, it was something easy for the military police to check which they constantly did. And what they were checking was this: did the number plate correspond to the registration which was required to be carried in every vehicle. To drive a German Army vehicle, you had to have a license to drive that specific type of vehicle so one can imagine the Feldgendarmarie knocking on one’s car window and demanding, “license and registration.”
On German vehicles, the number plates were coded in the following way: WH (Wehrmacht, Heer (army)), WL (Wehrmacht, Luftwaffe), WM (Wehrmacht, Marine (navy)), or SS. Each license plate began with one set of these letters. These two letter combinations were followed by five to six numerals, usually divided into a group of two numerals followed by a group of three or four numerals. The first two numerals indicated which command the vehicle belonged to such as Army District, 10th U-Boot Flotilla, etc. and specifically what type of vehicle it was. The last three or four numerals comprised the actual code letters of the vehicle.
So a license plate on a German Armed Forces truck which began WH, belonged to the Army. The next two numerals would indicate what specific model of truck and to which type of unit such as a panzer or infantry division or Armee Korps it belonged to and the last three numerals would indicate which specific truck of a specific model it was. It was a bit more complex than this but this will give you a sense of what the number plates mean. Each type of vehicle would have its own code. So each type of truck made by Mercedes or Ford would have had a different designation. Ford’s German subsidiary continued to manufacture trucks for the German Army all through the war. German units tended to prefer Fords over Mercedes because the Fords were more durable and were easier to maintain.
Additionally, vehicles were marked with the insignia of the division and/or higher formation or ad hoc formation they were assigned to. Example: vehicles assigned to the 4 Armee during the invasion of France in 1940, had a ‘K’ on their vehicles which stood for ‘Kluge’. Günther von Kluge commanded 4 Armee. All German Army divisions had a distinctive symbol which they put on signs, equipment, vehicles, etc. Example: the elite Großdeutschland (Greater Germany) division had as its symbol a white silhouette of a German Army helmet (1935 pattern). A tank or other vehicle of GD (as it was abbreviated) would also have had a tactical symbol indicating which type of unit the vehicle belonged to: infantry, armor, medical, engineers, etc.
Every German military vehicle, tank, or plane was also, then and now, marked with a version of the Balkenkreuz, which is said to be the symbol of the Teutonic Knights, a Germanic Catholic military/religious Order which conquered and ruled parts of Prussia and Eastern Europe in medieval times.
Source: Wehrmacht Camouflage and Markings 1939-1945 by W.J.K. Davies and Wehrmacht Divisional Signs 1938-1945 by Theodor Hartmann. I give both these books three stars. Lots of people are writing about stuff like this on the internet but for purposes of exactitude, one should assume that much of the detailed information people post on the net is incorrect.
Information on the Teutonic Knights can be found at the Catholic Encyclopedia.