Mac OS:Audio setup

To setup the outputs on Mac OS you have to open the Audio MIDI Setup.This defaults to being in the Utilities folder. Once open, choose the Audio Devices tab

Default inputs and Outputs
These are the default inputs and outputs that the Mac OS will read and send signal from. When you launch a DAW program such as Logic or Pro Tools, the internal hardware driver settings of the software will (usually) take precedence over these settings.

Clock source
This is used to determine what device is the digital clock source.