If a fuel line runs lower than the top of the tank and breaks, the tank can siphon out through it. With petrol, that fills the bilge with fuel and vapour. An anti-siphon valve sits where the fuel leaves the tank and stays shut until the engine's fuel pump pulls it open.
On US petrol boats it's a rule. 33 CFR 183.568 asks for each fuel line from the tank to the carburetor to run above the tank top, or to have an anti-siphon device or an electric fuel stop valve. A third option uses manual shutoff valves where the tank top sits below the carburetor inlet.
What a surveyor checks
The fuel line traced from the tank's withdrawal fitting to the engine, and its height compared with the tank top. Which of those protections is fitted, if any. Signs it's been removed or bypassed, which owners sometimes do when an engine starves. The fuel hose's type and age alongside.
In the report
"Fuel line from the starboard tank runs below the tank top to the engine. No anti-siphon valve or fuel stop valve fitted. Fit one before the vessel is next used." See missing anti-siphon valve.