A mechanical pressure independent control valve (PICV) combines a flow regulator, a differential pressure controller, and a control valve. Together they hold the required water flow regardless of changes in system pressure — within an allowable range, which is the part that matters in the field.
The differential pressure controller is typically a spring and diaphragm. The diaphragm senses the pressure on each side of the control valve and the spring pushes back against it, so the controller throttles itself to keep a constant pressure drop across the valve as system pressure moves around it.
A PICV helps contractors and engineers at selection, because it only requires matching the flow rate to the coil rather than determining Cv values and matching pressure drops. But the larger impact is the simplicity of the installation: a companion terminal balancing valve is typically not required, and the testing and balancing labor at the terminal is reduced to the testing rather than the balancing portion of the work. Belimo is explicit that circuit setters and automatic flow limiters must not be used with a PIQCV. Riser and system balancing can still apply.
This changes what the balancer does at each terminal. The P/T ports read P₁ − P₃, the differential available across the valve. Confirming that reading sits inside the valve's published range is what proves the valve can hold its preset flow. It is not a flow measurement, because with a PICV the flow comes from the preset rather than from a measured pressure drop.
The figure below is a Belimo ZoneTight PIQCV, which implements this as a differential pressure regulator in series with a characterized ball valve, with flow set by a clip position rather than a Cv selection. Move either pressure and watch where the differential goes.