Comment by BrandoElFollito

Usually an identity card. In the EU this is an authentication mean but in order to be liable you must be present with the card at transaction time (i.e. a scan is not enough).

Then you have solutions of increasing robustness such as certificates for e-signature.

The national "id" (of there is one) is just to make it easier to find you. Poland has one, France does not have any for instance.