Devices that support an external voltage reference enable you to maximize the resolution of your device. If the voltages you want to generate do not exceed a certain level and you can supply an external reference voltage at that level, you achieve your device's maximum resolution. The external reference voltage settings are available as a Channel property in the Analog Output»General Properties»DAC»Reference Voltage.
You also can use external reference voltages to apply a gain to a DC voltage or to a time-varying waveform. For example, set your external reference voltage level to 1.0, and write a sine wave buffer with values from -1.0 to +1.0 V. When you apply an actual reference voltage of 2.0 V, your signal jumps to ±2 V in amplitude. Increasing the reference voltage level to 3.0 again jumps the signal to ±3 V. Applying a reference voltage level of 0.0 V immediately flat-lines your time-varying voltage signal at 0.0 V.
The terminal you use for external reference sources varies depending on your device.