I have never used this feature, but my interpretation of the manual tells me that it is used to display a video source when one of the built in sources (like Tuner/Network/iPod/USB/M-Xport) is selected. CD apparently is available also, but only when an HDMI is not assigned. Not sure if they mean assigned to CD or something else.

But the real question is, what are you trying to do? If you are trying to play something like Sonos with CBL/SAT/AppleTV/BD etc, there is a better way to do it.