Right click on SmartIO component, choose 'Find Example Projects', you will find example project/code to use the SmartIO peripheral available on PSoC4S devices.
This component provide a programmable glue logic placed between the IO cell and the HSIOM/DSI/GPIO Mux, and are available for use in LPM. The glue logic is composed of eight 3-to-1 LUTs and a limited ALU block known as the data unit (DU). The glue logic can be used to route signals through the fabric and drive them either out of the chip through the IO cells or back into the chip to other internal destinations. It can be used to implement simple combinatorial and sequential logic meant to operate on the signals close to the IO cells. It is not intended to be used for UDB-like functionality, in the sense of constructing solution components.
One example: My customer have ever asked me to reverse the UART TX/RX logic inside PSOC, then they can remove some external logic device but SCB component itself cannot support this feature. SmartIO is being used to realize the reverse between SCB component and GPIOs.