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. This is the bracket that allows for installation of the diffuser on 02-07 WRX and STi. Also needed are the 4 bolts and Cusco Rear Subframe bar.