What is HDMI Audio Output "Expert Mode" exactly?

Have an XG1v4 connected to an AVR via HDMI.  In Device Settings > Audio > HDMI Audio Output there are 3 choices, Stereo, Auto Detect, and Expert Mode. 

Can someone please explain what Expert Mode is specifically?  It doesn't appear to do anything different to Auto Detect, passing DD 5.1.   What exactly should I see that's different?