In a direct acting valve, the main body or poppet is moved using magnetic force generated by the electrical coil to shift the valve. In a solenoid-pilot operated valve, the main body or plunger is moved using force developed from the available pilot pressure. The coil in this case is used to control the pilot for the pilot piston.

Force is required to shift a pilot or solenoid-pilot valve. The shifting force must overcome the opposing force (often a spring holding the valve in one position.) The valve specification includes a minimum force required to force the valve to shift. The physical force achieved by a pneumatic valve is created when the pressurized media acts on the surface area of the pilot piston, creating force. The force is measured by the pressure (of the media) multiplied times the surface area on which the pressure is applied (F=P x A). The minimum pilot pressure is minimum amount of pressure required to create a force greater than the opposing force in the device, thus allowing the control device to function.

Expilot is a term used to describe a solenoid-pilot directional control valve utilizing an external pilot signal. A pilot signal, often referred to as the pilot pressure, creates the force required to do work in the main body pilot valve. Solenoid-pilot valves utilize an internal or an external source for its pilot signal. An internal pilot source utilizes the main body media as the source of the pilot signal. A valve that uses an external pilot source does not utilize the main body media rather requires a separate, external pressurized source for the pilot signal. An expilot design is common when the pressure of controlled media does not meet the minimum pressure requirement of the main valve or where the control media need to be separated from the main body media.

A solenoid-pilot directional control valve utilizes a combination of a direct acting solenoid and a pilot valve. The solenoid direct acting valve controls the flow of a pressured media going to the pilot valve. The pressurized pilot media pushes on the pilot piston which creates a force (F=P x A). That force is used to move the plunger in the pilot valve. Flow control occurs via the configuration of the plunger. The action of moving the plunger controls the flow of the media. The advantages of the solenoid-pilot design are high flow capability, high reliability, low power consumption and simplicity of piping/installation as pilot lines are integral to the valve. The disadvantage is that it requires a minimum pressure to shift the plunger.