Float valve
WARNING: This page is about a different car, the 2001 Saab 9-3. However, it is still accessible from the selected car via links, so may be relevant.
The float valve is connected to the EVAP canister by a pipe. The valve prevents the fuel reaching the EVAP canister in a liquid state. During refueling, fumes from the tank are pressed through the float valve and into the EVAP canister.
A spring-loaded float closes the valve when the tank is full or in the event of a rollover.