The first rule for using a Master Clock is that the source equipment MUST be locked to the Master Clock. If you just lock the DAC to the Master Clock, there will be 2 independent clocks in the system running at different frequencies – one in the Master Clock, the other in the source equipment (probably a CD player or sound server). Each time the clock edges drift past each other in time, the DAC will either gain or lose a sample, resulting in a click or transient “phasey” distortion.
If your source equipment has no word clock input or cannot lock to the Master Clock for other reasons, the DAC must be set to Audio Sync.
In the case of DACs with asynchronous interfaces (USB or Network), the DAC alone can be locked to the Master Clock while using this type of interface. An asynchronous interface controls the delivery of data to match the DAC’s clock.