There is actually 3 cords from my Deck. After checking the manual... one usb is for Apple, one usb is for Android, and the other audio jack is for input from any device with audio out. I had to go into settings to set-up the Deck to recognize my Android phone.
If you can find a manual for your Deck online it may have info on that. Or the USB cord is for or from something else. Good luck figuring out what your Mystery Cord is for or from.