It looks like I understand what is going on. I suggest you read about the difference between Assignee/Candidate User/Candidate Groups.
Here are a few starting points 1 and 2 and 3
Also, you can specify stefan in Assignee field of the “Rechnung erfassen”, deploy it, start a new instance and see that a task is assigned to stefan.
But again, I would highly recommend reviewing the links to understand the difference between these concepts.