In the context of hydraulic schematics, what does a downward pointing arrow usually signify?

In hydraulic schematics, a downward pointing arrow typically signifies a relief valve that is normally closed. This symbol represents the operational state of the valve, indicating that it is designed to remain closed under normal conditions and will open to relieve pressure when it reaches a predetermined threshold. The downward arrow visually communicates the valve's function in managing pressure within the hydraulic system by directing flow away from the system when it exceeds safe levels.

The other options do not align with this specific symbol. A normal valve or an open valve typically has different representations to indicate their functions—normal valves might be depicted without arrows, or with configurations that reflect their open or closed states. A filter, on the other hand, has its own unique symbol that denotes its function in cleaning fluids rather than managing pressure. Thus, the downward arrow's association with a relief valve is reinforced by the widely accepted conventions within hydraulic schematics.