PCIN port is cascaded P input from the previous DSP Slice. Input to add/sub. Avoids FPGA routing and provides a low latency path.
and PCOUT port is optional cascade P output.
So basically the PCIN is a dedicated route to the PCOUT of an adjacent DSP48 slice. Dedicated cascade routes cannot be accessed in fabric.They cannot be connected to constant values or FPGA fabric, such as LUTs for flip-flops. The dedicated routes can only be connected between adjacent DSP48 slices.
Unused input cascade pins should be tied to '0' so that the tools will know that they are unused.Unused output cascade pins should be left unconnected so that the tools will know that they are not being used.