Check whether the certificate with FINGERPRINT (SHA-1 hash of the entire X.509 certificate blob) is valid or not by consulting the CRL responsible for this certificate. If the fingerprint has not been given or the certificate is not know, the function inquires the certificate using:
S: INQUIRE TARGETCERT C: D <DER encoded certificate> C: END
Thus the caller is expected to return the certificate for the request (which should match FINGERPRINT) as a binary blob. Processing then takes place without further interaction; in particular dirmngr tries to locate other required certificate by its own mechanism which includes a local certificate store as well as a list of trusted root certificates.
The return code is 0 for success; i.e. the certificate has not been revoked or one of the usual error codes from libgpg-error.