Definition
Under the UN Convention on the Law of the Sea, every state sets the conditions for granting its nationality to ships, and there must be a "genuine link" between the state and the ship (article 91). A ship sails under one flag only and may not change it during a voyage except on a real transfer of ownership or registry (article 92).[1]
Article 94 lists the flag state's duties: to exercise jurisdiction and control over its ships in administrative, technical and social matters — construction, equipment and seaworthiness, manning and crew training, use of signals and prevention of collisions — and to keep a register. In practice flags delegate much of the surveying to classification societies acting as recognised organisations.[1]
The flag decides which approvals the ship's equipment needs, which crew certificates are valid and which port state control regimes target her. Tell suppliers the flag when you order safety equipment or certified spares.
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Sources
- [1] United Nations — UNCLOS Part VII (High Seas), articles 91–94 — https://www.un.org/depts/los/convention_agreements/texts/unclos/part7.htm
A working definition for buyers, not legal advice — the contract, the convention text and your club or class have the last word.