Is "Certificate of Class" a statutory certificate or mandatory certificate ?
Let us first understand what are statutory certificates and what are mandatory certificates.
Statutory certificates are required by law. When we say LAW, SOLAS or MARPOL is not a law. It becomes law when SOLAS, MAPROL or any other convention is incorporated in the local laws of a country.
For example, SOLAS and MARPOL have become law in India because these IMO conventions are incorporated in the the Indian merchant shipping act which was passed by the indian constitution.
Mandatory certificates may not be required by law of the flag of the ship but are required for trade of the vessel. For example, it is not statutory for singapore flag vessel to have COFR but it is mandatory for trade in US waters.
Now lets come to certificate of class. As per SOLAS Ch II, ships must be constructed
either as per the standards of a classification society or
as per the applicable national standards of the flag state equal to the standards of the classification societies.
I am not aware of any flag state that has their own standards of construction of ships. Flag states require the ships to be designed, constructed and maintained as per the standards of classification societies.
This makes the "Certificate of class" a statutory certificate.