what is the HDMI out typically used for on an audio receiver
It's used as the output to the HDTV. The receiver (better ones!) have a few HDMI inputs and you select that input with your receiver control that receiver control helps reduce the number of remotes we collect as we use that for source selection and volume control. Many folk opt for a Logitech Harmony to drive the remote controls down to one (the Harmony.)
I have a question. You don't have to answer. Most folk buy these from a store or such and are they giving you no answers?