Community Identification Number
The Official Municipality Key, formerly also known as the Official Municipality Characteristic Number or Municipality Code Number, is a number sequence for the identification of politically independent municipalities or unincorporated areas. Other classifications for the identification of areas include postal codes, NUTS codes or FIPS codes.
The municipality key consists of five digits in Austria, which are generated as follows: The first digit designates the number of the Austrian state, the second and third digits designate the district, and the fourth and fifth digits designate the municipality.
3 25 21: Rappottenstein
See Code officiel géographique.
In Germany the Official Municipality Key serves statistical purposes and is issued by the statistics offices of individual German states. The municipality key is to be indicated in instances such as changing residence on the notice of departure or registration documents. This is done at the registration office in every town's city hall.
The municipality key consists of eight digits, which are generated as follows: The first two numbers designate the individual German state. The third digit designates the government district (in areas without government districts a zero is used instead). The fourth and fifth digits designate the number of the urban area (in a district-free city) or the district (in a city with districts). The sixth, seventh, and eighth digits indicate the municipality or the number of the unincorporated area.
...
Wikipedia