The registration certificate part 1 (Zulassungsbescheinigung Teil 1) contains detailed information about a vehicle’s technical information, and any modifications that have been made. It also contains the HSN TSN numbers which you need in order to get insurance quotes.
When you drive your vehicle, remember to have the registration document with you. In the case of a traffic check, for example, both your driving licence and registration document have to be presented.
When doing a general inspection (TÜV / Haupt-Untersuchung) of your car, you’ll get a stamp in your document that you passed the inspection.