Class is the vessels *Classification society*. These organizations provide expert design, maintenance, safety and compliance to regulations and rules/requirements and inspections service to the vessels owners, insurance companies and any group with an interest in the vessels operation.
There are 7 major ones, Lloyd's Registry, British, Det Norse Verites, Norway, American Bureau of Shipping, German Lloyd's, NKK, Japan, Hellenic, Greek, Bureaus Veritas, French and many other small ones, most usually associated with a country. These societies employ marine/ship surveyors, and many types of engineers and experts in many fields of ship design, building and operation. They work together with naval designers and shipyards to make sure the vessel to be built is well designed for its purpose and full fills all international safety, structural and operations requirements.
After the vessel is certified and all certificates are issued showing the vessel is in good order and full filled all requirements and is in operation they set up regular inspections and surveys of the vessel to make sure all international rules and regulations are being follow, proper maintenance and inspections and compliance records are in order. The class certificates are proof to all inspections groups, including Coast Guards, that the vessel is ok!
With cases like this with the infinity, where there is some damage, the class works with the owners, shipyard, various surveyors to set up the repair requirement. However the class requirements for the repairs is the final one. They have many detailed requirements for repairs. These include the proper grade of steel, size plate inserts, proper weld type and weld passes. All equipment and parts being used must pass class inspection and requirements.
Anything I missed Truck1?
AKK