Dig Master to L-R: When engaged, audio returned on digital return 17-18 will be sent directly to main mix L-R. You could essentially use this as an extra stereo bus to route audio back to the desk over and above the 16 analogue channels, note; there is no level control when doing this so the level of this bus will need to be tweaked in your DAW.
Dig Master: This is a feature of the monitoring section of the ZED-R16. When engaged the control rooms monitors and headphones will be listening to the 17-18 digital return only. Some examples for use are:
1) If you are bouncing down a 16 channel analogue mix into your DAW and want to monitor the digital return make sure your new audio track has its output set to 17-18. You will now be listening to the digitized return from your DAW including any digital processing plugins you may have added to the track, all in near real-time.
2) Listen to a mixdown without having to forfeit channels on the desk, potentially having to zero down EQ's, pans etc.