If you turn the two letters on their sides, you will see that the outlet of an “S” trap points down and the outlet of a “P” trap goes sideways. An “S” trap is illegal since it creates a perfect syphon and while it will drain the sink quicker, it will also draw the water for the trap seal out of the trap and result in minimal or no seal.
Towards the end of their life here, there were many interesting variations trying to minimize the effects of the syphoning, since the syphonage could not be eliminated. Bag traps and internal baffles were used to try to gt air through the trap and break the syphon before too much water was lost. Probably one of the last variations to try to keep them functional was the crown vent at the top of the return bend. But this location allowed debris to enter the vent and plug it.