When using a USB interface to transmit and receive MIDI data between a computer and an external MIDI device, many Owner's Manuals instruct the user to set the 'MIDI IN' to 'USB IN 0-1 and 'MIDI OUT' to 'USB OUT 0-1'. However, if another USB device is already plugged into the computer, this configuration may not work.
The reason is that the first number in '0-1' indicates a specific USB jack and the second number indicates the MIDI port the USB device is using. For example, if another USB device is already plugged into the computer when a USB-MIDI device is added, the USB-MIDI device would be '1-1'. If two USB devices are already plugged into the computer when a USB-MIDI device is added, the USB-MIDI device would be '2-1', etc.
The second number may also be different, if the USB-MIDI device can use mutiple MIDI ports; a device using one port may show as '0-1', while a device using a second port on the same USB connection would show as '0-2'.