When the act on behalf of International Maritime Organization member states they have to comply with the RO code.
The RO Code provides flag States with a standard that will assist in achieving harmonized and consistent global implementation of requirements established by the instrument of the International Maritime Organization (IMO) for the assessment and authorization of recognized organizations (ROs)[4] Classification societies set technical rules based on experience and research, confirm that designs and calculations meet these rules, survey ships and structures during the process of construction and commissioning, and periodically survey vessels to ensure that they continue to meet the rules.
In the second half of the 18th century, London merchants, shipowners, and captains often gathered at Edward Lloyd's coffee house to gossip and make deals including sharing the risks and rewards of individual voyages.
It did not take long to realize that the underwriters needed a way of assessing the quality of the ships that they were being asked to insure.
The condition of the hull was classified A, E, I, O or U, according to the state of its construction and its adjudged continuing soundness (or lack thereof).
The adoption of common rules for ship construction by Norwegian insurance societies in the late 1850s led to the establishment of Det Norske Veritas (DNV) in 1864.
RINA was founded in Genoa, Italy in 1861 under the name Registro Italiano Navale, to meet the needs of Italian maritime operators.
The largest are DNV, the American Bureau of Shipping, Nippon Kaiji Kyokai (ClassNK) and Lloyd's Register.
The classification rules are designed to ensure an acceptable degree of stability, safety, environmental impact, etc.
In particular, classification societies may be authorised to inspect ships, oil rigs, submarines, and other marine structures and issue certificates on behalf of the flag state.