Final answer:
Each plumbing fixture trap is protected against siphonage and back pressure by a vent system, which balances air pressure to ensure proper function and safety.
Step-by-step explanation:
Each plumbing fixture trap shall be protected against siphonage and back pressure by a vent system. The purpose of the vent system is to maintain a balance of air pressure within the drainage system. This balance prevents the water in the trap from being siphoned out, which would allow sewer gases to enter the building. The vent system is a crucial component of modern plumbing that ensures the traps function properly, safeguarding both the efficiency of the plumbing system and the health of the building's occupants.