The output relay switches ON when the 2N Net Audio Decoder unit is used in active session. Typically you can use it by switching on a power amplifier in order to achieve higher overall output, for example 200W etc.
The function of switching ON and OFF this relay should be set individually for every 2N Net Audio Decoder unit in the 2N IP Audio Control Panel, section Speakers, Click on particular speaker and on the right side under Audio parameter is the checkbox "Switch ON relay while transmitting".