I'd suggest you to check these following documents. In them you can find information about the Arduino Expansion Board and the USB ports as well:
Edison Arduino Expansion Board Schematics (check page 9)
Also take a look to this thread about the USB Host port, it might be of your interest: How do we use the host USB port ?
Regarding the switch between the USB ports, it is used to select the USB mode of the USB OTG port of Edison. You can only use one of those modes at a time, check the page 9 in the schematics document above.