The port 'S' in a ball valve block refers to a physical signal input that can be used to control the ball displacement. A positive signal at the physical signal port S opens the valve. The S port in the 3-way directional valve also refers to a physical port that can be used to control the spool displacement similar to the S port in the ball valve block. You can refer to this example model to get an idea of what input can be used to the S port:
https://www.mathworks.com/help/physmod/hydro/examples/hydraulic-differential-cylinder-with-3-way-valve.html
