One is HDMI-IN where you'd connect a game box, DVD player, laptop etc. The other is HDMI-OUT where you'd connect a TV or external monitor. Check the back panel view here to see which is which.    
I understand what the HDMI ports are used for, but not necessarily so when it comes to an all in one product.    Can the be used for both input or output?  Can I add additional computer's HDMI output to be displayed via the AIO product's display (i.e. as input)  or are they only there to display the AIO product's HDMI output on other monitors (i.e. as output)?
What can the 2 HDMI ports be used for on the 7700 AIO product?