What is REC out and what is it for?

REC OUT
Connect to the recording input of a CD recorder input, tape deck or other compatible recording devices etc.

This output provides a constant high-level output that does not fluctuate with the volume of the main unit. For this reason, the AXR85/100 cannot be used as a Preamplifier only and the Rec Out can’t be connected to a Subwoofer.

Rec-out does not behave like a Pre-out.
Rec-out volume does not fluctuate with volume controls, unlike a Pre-out.