It's for hooking your cell phone to display on the radio. Thor has a "how to" video about it on their YouTube channel. You have to have a compatible phone.
iPhone has a dual jack lightning attachment for screen mirroring (HDMI and USB) plug the HDMI into the port to screen mirror the phone to the radio screen...run maps, etc, and plug the usb into that port to charge the iPhone while it's mirroring or it'll die pretty fast. Samsung...not sure it's inferior...LOL
When I use the HDMI jack with my I Pad 3 I do not get full screen just a 3/4 screen in center of display. Is there a way to make it full screen display?
I may just mount the I Pad since its display is larger anyway.