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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I just found a service manual online, and in this one there are _three_ lines going from the fuel tank to the engine bay. The third line seems to connect to the "fuel check valve", and is labeled "evaporation tube". What's the deal here? Just avoiding venting the fuel fumes to the atmosphere?
 

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