There are nominations of candidates in Bundestag and European elections which have to be signed by a fixed number of persons entitled to vote. By signing, these people show that they are in favour of the nomination. This is to ensure that only serious nominations of candidates can be voted for which have a reasonable number of supporters amongst the electorate.
In the case of established political parties it can be assumed that there is a sufficient number of supporters among the voters. Therefore, they do not have to submit supporting signatures.
Last update: 17 January 2019