Among the features that make AXE I/O the definitive guitarist interface is the Re-Amplification feature. The “AMP OUT” is a fifth physical output which is independent from main Output 1-2 and 3-4. With this output, you can feed a DI recorded guitar to external analogue stomps or a real amplifier, still being able to monitor the session from main Output (1-2). The external stomps (or the miked amplifier) will be connected back to Input to return into the DAW on a newly recorded track.
What I used the 'amp out' for was recording my fx/amp (external gain-based fx) for tonex capture without any signal intererence.