Associated descriptor fields: iSerialNumber (in the Device Descriptor); and the string in the String Descriptor that it indexes.
Suggested default: check the box
This checkbox determines whether the device will report a unique serial number. If you check the box,the MSP430 USB API will automatically read the die ID unique to a particular physical MSP430 device, and form a unique serial number from it. This will be reported to the USB host, as the USB serial number. If unchecked, no serial number string will be reported.
Generally it is a good idea to report a serial number. (For more information on the tradeoff between these options, reference the Programmer's Guide for the associated API stack.)