A pressure reducing valve lowers the downstream pressure.
It is often used to limit pressure in pilot operated, directional control valves. The pilot pressure that is required for these valves is usually well below the maximum system pressure.
A pressure reducing valve may also be used to limit cylinder force or limit brake application or release pressure in some mobile machines. These lower pressures help prolong component life. It is desirable overall, to operate sub-circuits and actuators at the lowest pressure that still allows the work to get done.
Here are a few examples of simple schematic symbols for a pressure reducing valve.
Notice that in all cases, the main poppet arrow indicates that the valve is normally open.
Here's a side-by-side view of a more detailed pilot operated pressure reducing valve symbol and cross-section.
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