What is the meaning of DWDJM on the special card for social insurance contributions?

The DWDJM on the card is the registration code.

The so-called registration code is in the unit for the first time for social security account opening, will tell the user a unit of social security number, this number, is the user's unit of social security registration code; insurance registration card on the code, usually 12 digits, the first three is is the province (municipalities, autonomous regions) of the code, the next three is the city (district) of the code, and the last six digits of the code of your unit.

The registration code can be found directly at the social security center.