vcTypeMetadata

SD-JWT VC: Credentials MAY encode Type Metadata directly, providing it as "glue information" to the Consumer.For JSON-serialized JWS-based credentials, such Type Metadata documents MAY be included in the unprotected header of the JWS. In this case, the key vctm MUST be used in the unprotected header and its value MUST be an array of base64url-encoded Type Metadata documents as defined in this specification.

Defined as a String here, so client applications can parse to appropriate types.