With the Certificate of Conformity (COC), the manufacturer proves that the vehicle complies with the applicable EU standards or has EC type approval. In German it is also known as a certificate of conformity or EC certificate of conformity. It contains all registration-relevant data and makes it much easier to register the vehicle in EU countries.
In the COC papers you can find various information on, for example, permissible tire/rim combinations, vehicle dimensions or the exhaust gas classification of your vehicle.