A backflow is required for any sprinkler system supplied from culinary water (drinking water). If you are not familiar with Backflows you need to do some research for your particular area since there are different requirements for different areas.
Backflow Definition
Undesirable reversal of flow of a liquid, gas, or suspended solid into the potable water supply. A backflow preventer is designed to keep this from happening.
Why Do I Need A Backflow Protection Device?
A Backflow Preventer protects drinking water from becoming contaminated in the event of a loss in pressure from the supply.
Backflow Preventers Commonly Installed on Sprinkler Systems
PVB, Pressure Vacuum Breaker
The PVB is my favorite backflow preventer to use. It is easy to install, meets the requirements for most sprinkler systems and water departments, and it is also the least expensive. As long as you follow the installation guidelines/instructions provided by the manufacturer, this backflow preventer provides a great line of defense. The only form of backflow that the PVB does not protect against is back pressure. It is never a good idea to use a PVB in a system that has any kind of pump or has any kind of secondary water supply.
RP, Reduced Pressure Assembly
The RP is the best protection available for sprinkler systems connected directly to a pressurized culinary / potable water system. The RP backflow preventer is also the most expensive option in most cases.
To Learn More Check Out This You tube Video "What is Backflow"