Fixtures, Fixture Fittings, and Appliance Connections
Three things get tested together
Fixture questions blend the fixture, its fitting, and its appliance connection. Every potable connection needs backflow protection matched to the hazard: hose bibbs need vacuum breakers, and water-using equipment with a health hazard needs an air gap. A dishwasher drain hose requires a high loop or an air gap. A clothes washer standpipe must be 18 to 42 inches above the floor with a trap below the standpipe. A water heater T&P discharge stays full size, drains by gravity, and terminates within about 6 inches of the floor or to the exterior.
Setting and clearances
Water closets need the rough-in matching the fixture (commonly 12 inch), side clearance, and a flange at finished floor level. Lavatories and sinks must be secured and sealed to the wall and accessible for cleaning. Floor drains need trap primers where the trap could evaporate. Indirect waste receptors receive ice makers, steam tables, and dishwashers through a proper air gap - never a direct connection.
Traps
Reducing or capping a T&P discharge line, running a washer standpipe above 42 inches, using a fixture drain as a vent, forgetting the vacuum breaker on a sillcock, and treating an indirect waste as a direct connection.
Rule of thumb
Before choosing a fitting, ask where the air gap is. If the answer is "there isn't one," the connection is wrong regardless of how neatly it is piped. Then verify the rough-in dimension and standpipe height against the manufacturer's installation instructions, which the code makes part of the requirement.