Each FT232R USB UART IC and FT245R USB FIFO IC has a unique identification number burnt into it during manufacture, the FTDIChip-ID.
The FTDIChip-ID is readable over USB, but cannot be altered by the end user. This feature provides a way of tying application software to a specific device with a high level of security.
The FTDIChip-ID feature can be used without affecting any of the other device features.