The concept of Identity proofing is the process of getting user attributes into the vIdP. Initially when a user account is been created, it only consists of a username. In order to attach attributes, such as a name, birthdate or nationality to that user account, a user sends an ID Proof to the vIdP. The vIdP can then validate the proof and attach the attributes to the user account.
ID Proofs may take many forms and be anything ranging from X.509 certificates and identity tokens from other IdPs to self claimed (non-validated) user attributes.
The concrete vIdP deployment is responsible for defining which types of ID proofs to accept, as described in the vIdP deployment section.