uni-assist e.V. does not cooperate with commercial agencies in or outside of Germany to mediate the placement of applicants at German universities. Some institutions claim to be connected to uni-assist. Official information on applying to uni-assist universities is available only on the websites of the universities and on https://www.uni-assist.de/en/.
Certificate of enrolment ("Immatrikulations-Bescheinigung")
A certificate in which your university attests that you are enrolled as a student.