A usual usb-cable has the familiar plugin interface on a computer, to be linked to a similar plug of a cable inversely. Certain devices have a different plugin opportunity for the connecting cable, which can be seen at http://superuser.com/questions/856297/monitor-with-usb-upstr... . This photo says it goes into a monitor, but various types of outside storage drives also use this plugin to differentiate it from the computer's own usb port. This way, plugging is unambiguously determined.