XR900 is a smart receiver,it need to work with smart thermostat(XR900RF) together suitable for home or office. Receiver and thermostat will communication through build-in RF model.It controls the relay according to the control instructions received from the smart thermostat(XR900RF)  or outputs the opentherm data through the opentherm communication port,and send some data from Opentherm Boiler to smart thermostat(XR900RF)