What is a "backflow prevention device" intended for?

"Backflow" causes a "cross-connection". A cross-connection occurs when a faucet, hose, or other plumbing fixture that provides clean water is mistakenly connected to dirty water. An example would be a hose connected to a faucet with clean water that is left submerged in a mop bucket of dirty water. Or a sink faucet that is too low and becomes submerged when the sink is full of dirty dishwater. Backflow occurs when the dirty water from the mop bucket or sink full of dirty dishwater flows through the hose or faucet and into the clean water supply. This can happen due to natural changes of pressure within the building's plumbing system. To prevent backflow and cross-connection, there should never be any direct connections between clean water and dirty water. The two most common ways to prevent backflow is an "air-gap" or a "backflow prevention device". An air-gap is simply allowing a space of air between the clean water and the dirty water, such as a high faucet that cannot become submerged when the sink is full. Or never submerging a hose into the bucket of mop water. A backflow prevention device is a plumbing fixture with valves that keeps water moving in only one direction - and never backward.