Common cross-connections that should be protected against backflow:
Toilets can be protected against backflow with an anti-siphon ballcock assembly installed in the toilet tank. This will protect against backsiphonage. The ball cock can also serve as a thermal expansion relief device, if equipped with an auxiliary relief valve. The relief valve should govern the preset pressure to 80psi or less.
To prevent contaminated water from being siphoned back into the drinking water, a hose bib vacuum breaker is needed for laundry sinks. The vacuum breaker attaches to the threaded end of the outside water faucets (sill cocks) and most hose bibs come have built in vacuum breakers.
Outdoor faucets, aka hose bib, are what you attach the garden hose to. You can easily prevent contamination by installed a hose bib with a vacuum breaker. The most common way to contaminate the water supply happens when one end of the hose is attached to an outdoor faucet, and the other end is connected to an aspirator type bottle, insecticides or other chemicals in the aspirator bottle can be siphoned back into the drinking water supply.
Lawn Irrigation System
Lawn irrigation systems need a vacuum breaker backflow preventer to protect against lawn and pesticide chemicals being drawn in from the lawn and back into the drinking water supply.