USB Devices that support both full and high speed modes must have a device_qualifier Descriptor. When the USB device switches its speed mode, values in some fields get changed in device descriptor. The device_qualifier descriptor provides information only about those field values that get changed when device switches to an alternate speed mode.

For example, if the device is currently operating at full-speed, the Device Qualifier returns information about how it would operate at high-speed and vice-versa. The fields for the vendor, product, device, manufacturer, and serial number are not included. This information is constant for a device regardless of the supported speeds.