The USB Mass Storage Class - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 49

iProduct. Index to a string descriptor that contains a string describing the
product. This value is zero if there is no product string descriptor.
iSerialNumber. An index that points to a string containing the device's
serial number. Virtually every mass-storage device must have a serial number
whose final 12 characters differ from the final 12 characters in the serial
number of any other device with the same Vendor ID and Product ID. The
only exception is devices that use the CBI protocol, which don't require
serial numbers.
bNumConfigurations. The number of configurations the device supports.
This value is almost always 01h.
Configuration Descriptor
Each device has at least one configuration that defines the device's features
and abilities. Multiple configurations are allowed but rare. The configura-
tion descriptor contains information about the device's use of power and the
number of interfaces supported. Table 3-2 shows the fields in the configura-
tion descriptor. Here are more details about the fields and how they're used
in a mass-storage device:
bLength. The length in bytes of the descriptor. Always 09h.
bDescriptorType. The constant CONFIGURATION (02h).
wTotalLength. The number of bytes in the configuration descriptor and all
of its subordinate descriptors. The subordinate descriptors include interface
and endpoint descriptors but do not include string descriptors.
bNumInterfaces. The number of interfaces in the configuration. The mini-
mum is 1. A device with multiple interfaces can perform multiple functions,
such as mass storage and human-interface device.
bConfigurationValue. Identifies the configuration in Get Configuration
and Set Configuration requests. Set to 01h for the first (or only) configura-
tion.