Float valve
WARNING: This page is about a different car, the 2003 Saab 9-3. However, it is still accessible from the selected car via links, so may be relevant.
There is a line connecting the float valve with the emission canister. The float valve prevents fuel in a liquid state from reaching the evaporative emission canister. When refuelling, the vapor in the tank is forced through the float valve and into the evaporative emission canister.
A spring-loaded float closes the valve when the tank is full or if the car rolls over.