VID, DID, Subsystem Vendor ID, Subsystem Device ID

The Subsystem Device ID and Vendor ID registers are part of the PCI Configuration registers. These are
usually used when a PCI device is used as part of an add-in card. For example, if you are using the CY7C09449PV PCI Dual-port on an add-in board that your company manufactures, the PCI dual-port itself has the Vendor ID and Device ID provided by Cypress Semiconductor.
However, the subsystem Vendor ID and Device ID would be specific to your company. This is used to uniquely identify the add-in card or sybsystem that the PCI-DP resides within. Using these two registers, your computer can then differentiate between two different
boards made by different companies that may both contain the CY7C09449PV chip. Otherwise, your computer would not know which drivers to use with which board.

We do not assign the Subsystem Vendor ID. This is a Vendor ID assigned to your company by the PCI-SIG (Special Interest Group). The PCI-SIG is the only body officially liscensed to give out IDs. You need to be a member of PCI-SIG to request your own ID. Information
about joining PCI-SIG is available here: http://www.pcisig.com. The
Subsystem Device ID is then whatever you choose for that specific board.