[standards] there are more options for identifiers (standards section)

[Sakurann]

a good starting point might be to separate whose identifiers we are talking about:

• for issuers and verifiers, https url, x509 are common (and DIDs)
• for holders, some implementations use cnf claim, without explicit user identifier (which is sufficient for batch issued credentials) (and DIDs) (and URLs if holder is a legal entity, but I think this is still WIP, so can be added later)

https://openid.github.io/OpenID4VP/openid-4-verifiable-presentations-wg-draft.html#section-5.7 is one of the relevant sections just to showcase different options