A vehicle ownership document is a vehicle’s proof of ownership.
It contains information about both the owner and the vehicle. However, it’s different from the vehicle registration document (Fahrzeugschein - Zulassungsbescheinigung Teil I).
Unlike the registration certificate, you’re not obliged to keep the ownership certificate with you at all times. In fact, it’s better to keep it safely at home. You only have to bring both certificates when you register or sell your vehicle. This is to prevent stolen vehicles from being driven or sold.