a/s means asymetric.
What that means (I think) is that the tyre has an outside edge that always has to be on the outside of the car. So you could happily swap your wheels around left to right, front to back, whatever, end they'll always be fitted correctly. PE2's are asymetric.
The other type is directional/rotational - where which edge is on the outside doesn't matter, but the tyre itself always has to rotate in a particular direction. If you wanted to swap a wheel from left to right, you'd need to get it turned around on the rim first. Toyo Proxes are rotational.