See Figure 1
A pressure-vacuum relief cap seals the fuel tank. Tightening the cap on the fuel filler tube forms a seal between them. The relief valves in the cap are a safety feature. They prevent possible excessive pressure or vacuum in the tank. Excessive fuel tank pressure could be caused by a malfunction in the system or damage to the vent lines.
The seal between the cap and filler tube breaks when the cap is removed. Removing the cap breaks the seal and relieves the fuel tank pressure. If the filler cap needs to be replaced, only use new filler cap with the same pressure rating.