The STI Rear Diffuser came standard on the 06-07 USDM STi. It channels the air underneath the car to help with high speed stability. You can either purchase the diffuser alone if your car already came with one from the factory or you can purchase the additional hardware in order to bolt it onto your vehicle, such as the Cusco Diffuser Mount and Hardware.
The hardware you will need when installing this on a WRX or 04-05 STI is as follows:
1x CUS 672 487 S
1x CUS 673 487 A
4x SUB 901000365