The output matrix of the iD Application can be configured in many ways to give you a great deal of flexibility with your outputs. There are three different things you can route to your outputs; Main Mix, Cue and DAW thru
This is the main output of the iD mixer, so any channel in the mixer that has its main fader turned up will be present in the Main Mix. This is typically what is used for the speaker output. The Main Mix level is what is visible in the hardware meters and the meters in the master section of the iD app.
The cue sends can be found on every channel as a small fader. The cue sends on each channel are summed to the Cue Master found in the Master Section, the output of which is the Cue Mix. You can then send this to either the headphones or the speakers. This is useful for foldback when your artist (or yourself) are overdubbing some previously recorded tracks.
DAW Thru entirely bypasses the iD mixer application and takes the outputs from your DAW straight into your outputs. This also bypasses the volume control so your volumes are all controlled in your DAW. If you select this without making sure you have a low enough volume in your DAW then you might get a very loud output.