The fuel tank has a breathing function so that air enters when the fuel level drops. But fuel vapor should not reach the open air if possible. A coal filter prevents this. It basically sucks in the fuel particles. However.... this filter will need to be purged or else every few thousand kilometers the filter needs replacing.

So it gets purged by sucking the air back into the engine so reversing the flow. Fresh air from outside to clear the filter. This happens at high vacuum, so when the engine is idling or driving with minimal gas. A vacuum valve operates this flow. The valve number is 928.110.429.08