The Hunter PRO Smart Controller is factory set to accept two different sensors (most controllers can only take one). We recommend a rain/freeze sensor and a flow sensor. All of our Smart controllers come installed with a rain/freeze sensor at no additional cost. The flow sensor is priced per site because two wires need to be run to its location and it has to be installed near the backflow.
The distance of running those wires and the work involved to accomplish that on each site may be drastically different.
The flow sensor measures water flow and reports leaks. Soil sensors are also available but are not ideal for use on every zone as a result of the way they work (just because the soil is wet in one area of a property does not mean another area is wet as well) but they are a good solution for specific zones.