[mickmate]

The two components (vent and rollover spill valve) are typically combined in one unit. The tank needs to be vented so it can let fuel out without causing a vacuum or collapse of the tank as fuel is pumped out. If the tank isn't vented the car won't run for long. The trick is to vent it without fumes. Rollover spill valve stops fuel running out in the unfortunate circumstances the tank and car are upside down.