The Valve Body: The brass body houses the internal mechanism—typically a spring-loaded diaphragm—that automatically adjusts to maintain a consistent, lower output pressure regardless of incoming pressure fluctuations.
The Red Cap: This is generally the adjustment knob or a protective cover for the adjustment screw. Turning this allows you to set the desired downstream (outlet) pressure.
The Pressure Gauge: This is attached to monitor the system's pressure, providing a visual reading of the water pressure currently in the line.