What's a "fuel check valve"?

I just dropped the fuel tank out of my 1990 1.2 litre Micra, and saw a line coming out of the tank that I don't know the purpose of. I recognize the filler tube, breather tube, feed and return, but what's the fifth one? My Haynes manual calls it a "fuel check valve" but doesn't go into any more details. So, what does it do?
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