What is a vent valve? on 2005 BMW 745i

I had two vent valves replaced. What is a vent valve? How much should the repair cost? Thanks for your help.

Asked by for the 2005 BMW 745i
Is there a part number on your repair order? I suspect the valves are related to the "emission control system". There are valves that allow fuel tank vapors to be drawn in to the engine under certain conditions to reduce pollutants being emitted into the air.
Where these vent valves replaced to solve a Check Engine Light issue?