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A CA-designation (e.g., "class 0" or "class 1") indicating how thoroughly the CA checked the validity of the certificate. Per X.509 rules, the "class" should be encoded in the certificate as a CP extension: the CA can insert an object identifier (OID) that designates the set of procedures applied for the issuance of the certificate. These OIDs are CA-specific and can be understood only by referring to the CA's Certification Practice Statement.
NIST SP 800-57 Part 2 Rev.1