With the QNX Neutrino OS, the USB enumerator attaches to the USB stack and waits for device insertions. When a device insertion is detected, the enumerator looks in the configuration manager's database to see which class driver it should start. It then starts the appropriate driver, which provides for that class of device. For example, a USB Ethernet class driver would register with io-pkt* and bring the interface up.
For small, deeply embedded systems, the enumerator isn't required. The class drivers can be started individually — they'll wait around for their particular devices to be detected by the stack. At that point, they'll provide the appropriate services for that class of device, just as if they'd been started by the enumerator. When a device is removed, the enumerator will shut down the class driver.
For more information about device enumeration, see the Controlling How Neutrino Starts chapter of the Neutrino User's Guide.