[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.