Mitwirkung
The duty to submit documents the authority needs for your benefit.
When the Jobcenter, Familienkasse or health insurer asks for proof, they set a deadline. Missing it risks disadvantages, up to the benefit not being paid for now. If you cannot get something in time, tell the authority before the deadline.
How it appears in the letter
»Bitte reichen Sie folgende Unterlagen bis zum … ein.«