The actuators could be air cylinders, could be rotary actuators... any air-driven device that requires air to be supplied to alternate ports in order for the device to cycle.
The 5/3 or 4/3 series are directional-control valves. The 5/3 and 4/3 body design allow compressed air to flow to one port of a double acting air actuator while simultaneously allowing air to exhaust from the other port on the same air actuator at the same time.
By shifting the internal flow paths of the valve, the 5/3 and 4/3 air valve sends compressed air alternatively to each of the two actuator ports and exhaust from the other, thus allowing the double acting air cylinder to function.
The valve shown in the following image has 5 air ports. It may be a 5/3, or it may be a 5/2 configuration. You cannot tell the difference from looking at the valve body. The valve schematic, which is typically shown on the side of the valve, is the only way you can determine if the spool is two position (a 5/2) or three position, (a 5/3) unless it is identified as such by the vendor.
Each of the three spool positions is selected to accomplish a desired result in the action of the air cylinder.
Since valves with three position spools are more expensive than their two position counterparts, the selection of a three position valve will be deliberate. The circuit designer will have a particular scenario in mind for the action of the air cylinder when the valve that controls it is shifted, and that circuit will require the selection of a specific three position valve to accomplish the goal.
A 5/3 or 4/3 valve will normally have two internal spring actuators that, when the valve is not being operated by an external valve actuator, shifts that valve spool to the center position automatically. It is normally when the 5/3 or 4/3 valve is "at rest" that the third of the three positions comes into play.
With this spool selection, the circuit designer has decided that when the valve is "at rest", it will be necessary to move the cylinder rod (and of course the end of rod tooling) by hand, or perhaps another operation will move the rod and tooling, and since there is no air on either side of the piston inside the cylinder, this can happen relatively easily, this is facilitated.
In this scenario the air circuit designer wants to have air to both sides of the air actuator when this valve is at rest.
The air actuator might be a rodded air cylinder, but it also might be a rodless type .
By exerting pressure on both sides of the piston inside a rodless ( band, magnetically couples or cable type) cylinder, the end of rod tooling can be held in one location. As an added advantage, if there are small leaks in the lines or through the seals of the rodless cylinder, a pressure center valve means that the air pressure will be maintained inside the cylinder regardless of small leaks.
Not all valve manufacturing companies offer three position valves, and those that do, not all of them offer all three of the possible valve spool configurations.
The 5/3 valve body ports are; one supply, two air cylinder and two exhaust ports.
The 4/3 valve body ports are: one supply port, two air cylinder ports, and one exhaust port.
The second digit in the valve designation indicates how many positions that valve can have. The 5/3 or 4/3 will have three positions.