A USB device sends a message to Windows at the expiration of its idle timer, asking Windows to put the USB port in a suspended state. If you disable USB selective suspend, Windows will return a failure to the driver when the driver asks Windows to put the USB port in a suspended state.
In my case, Windows is returning a failure to the driver, even though selective suspend is enabled. I have not found a good reason for this. Considering the minuscule amount of power that is wasted by not suspending USB ports, I'm not going to worry about it. Another one of Microsoft's unhelpful reports. That's why I forever have to 'explore other options'.