If you have imported a vehicle from abroad, in most countries you will need the Certificate of conformity to confirm, that it is a European model and can be driven in the traffic without the need for any modifications.
In the case you want to register your car in France and fields D1, D2, D3 and K are filled in the technical license, the COC may not be necessary, but the ANTS may sometimes ask for it additionally.
The registration process of an imported vehicle differs in individual EU countries. More information on registration in a specific country can be found here.