It is a matter of choice. Actually, most candidates are constituency candidates and they figure on the list. They have a list position.
There are a few exceptions. For example, the president of the Bundestag has no constituency because campaigning in a constituency would be so outspoken for his party that it would be detrimental to his continuing as president in the next legislature.
There are a few leading politicians who are only on the list, but most politicians are on both. They are constituency candidates where they live and they are on the party list of the state they belong to.