After five months of German occupation, the persecution of the Jews in Belgium started in the autumn of 1940. In October 1940, a start is made with the separation of Jews from Belgian social life. All Jews must first be registered and located for this purpose. However, the German occupier has a major administrative problem: Belgian legislation nowhere registers who is of Jewish origin.
The first German regulation therefore defines who should be considered a Jew based on biological descent: "Any person who has at least three Jewish grandparents is Jewish." In questionable cases, anyone who belongs or had belonged to the Jewish religious community is considered a Jew. Persons who meet this definition must register in a register of Jews at their municipality. "A total of 42,642 Jews over fifteen years of age register in their municipality.
In 1940 the Jews could not suspect that two years later the Germans would use the Register of Jews to track down and deport Jews. The ordinance of 1 June 1942 stipulates that during curfew Jews must be at their legal address, as noted in the Jewish Register. Thanks to the Jewish Register, the Sicherheitspolizei knows where they can arrest Jews during night raids.
Source- as an example: http://users.telenet.be/holocaust.bmb/ned/Brandel.htm