For CueServer models that have digital outputs, each output is designed to "sink" current to ground when the corresponding output is "on". When an output is "off", the port is not connected to anything.
This scenario allows the user to attach indicator LEDs, relays, buzzers and other similar low-current devices to CueServer by using the switched ground output ports. The device being controlled can be powered by any operating voltage (for example 5V, 12V or 24V) on one leg of the device and the other leg of the device can be attached to CueServer. Then, the power supply's ground is connected to CueServer's ground to complete the circuit. When an output on CueServer's output port turns "on", it completes the circuit to ground, which turns on the device (LED, relay, etc.).
Each digital output can handle an operating current of 500mA with a maximum voltage of 36VDC.