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 five different sources you can route to your outputs; Main Mix, Alt Speaker Cue B, Cue B 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.
Alt Speaker takes the signal from outputs 3+4 on your mixer. This is best used with a set of smaller speakers. Its always good practise to hear your mix on as many systems as possible to make sure they translate properly. Its great if your mix sounds good on your dedicated, expensive studio monitors but it's even better when your mix sounds great on small speakers like a vast majority of the public will be listening to your tunes on.
The Alt outputs on the iD22 allow you to do this without and unplugging and replugging of cables.
Cue A Mix:
The cue A 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.
Cue B Mix:
The Cue B mix works in the exact same way as the Cue A mix but instead receives sends from the CUE B faders. Having two cue mixes gives you a massive amount of flexibility when working in complex setups.
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 then you might get a very loud output.