The Device Descriptor (USB_DEVICE_DESCRIPTOR) is the root of the descriptor tree and contains basic device information. The unique numbers, idVendor and idProduct, identify the connected device. The Windows operating system uses these numbers to determine which device driver must be loaded.

idVendor is the number assigned to each company producing USB-based devices. The USB Implementers Forum is responsible for administering the assignment of Vendor IDs.

The idProduct is another 16-bit field containing a number assigned by the manufacturer to identify a specific product.

Offset	Field	Type	Size	Value	Description
0	bLength	uint8_t	1	Number	Size of this descriptor in bytes.
1	bDescriptorType	uint8_t	1	Constant	Device Descriptor Type = 1.
2	bcdUSB	uint16_t	2	BCD	USB Specification Release Number in Binary-Coded Decimal (i.e., 2.10 is 210h). This field identifies the release of the USB Specification with which the device and its descriptors are compliant.
4	bDeviceClass	uint8_t	1	Class	Class code (assigned by the USB-IF). If this field is reset to zero, each interface within a configuration specifies its own class information and the various interfaces operate independently. set to a value between 1 and FEh, the device supports different class specifications on different interfaces and the interfaces may not operate independently. This value identifies the class definition used for the aggregate interfaces. set to FFh, the device class is vendor specific.
5	bDeviceSubClass	uint8_t	1	SubClass	Subclass code (assigned by the USB-IF). These codes are qualified by the value of the bDeviceClass field. If bDeviceClass is reset to zero, this field must also be reset to zero. not set to FFh, all values are reserved for assignment by the USB-IF.
6	bDeviceProtocol	uint8_t	1	Protocol	Protocol code (assigned by the USB-IF). These codes are qualified by the value of the bDeviceClass and bDeviceSubClass fields. If a device supports class-specific protocols on a device basis as opposed to an interface basis, this code identifies the protocols that the device uses as defined by the specification of the device class. If this field is reset to zero, the device does not use class specific protocols on a device basis. However, it may use class specific protocols on an interface basis. set to FFh, the device uses a vendor specific protocol on a device basis.
7	bMaxPacketSize0	uint8_t	1	Number	Maximum packet size for Endpoint zero (only 8, 16, 32, or 64 are valid).
8	idVendor	uint16_t	2	ID	Vendor ID (assigned by the USB-IF).
10	idProduct	uint16_t	2	ID	Product ID (assigned by the manufacturer).
12	bcdDevice	uint16_t	2	BCD	Device release number in binary-coded decimal.
14	iManufacturer	uint8_t	1	Index	Index of string descriptor describing manufacturer.
15	iProduct	uint8_t	1	Index	Index of string descriptor describing product.
16	iSerialNumber	uint8_t	1	Index	Index of string descriptor describing the device's serial number.
17	bNumConfigurations	uint8_t	1	Number	Number of possible configurations.