Pressure reducing valve symbols look very similar to those for pressure relief valves. This is because the valves work in similar ways except that the pressure sense line feeds from downstream of the reducing valve instead of upstream as in the relief valve. This means that the reducing valve controls the downstream pressure rather than the upstream pressure. As such the reducing valve cannot generate a downstream pressure that is not there but it can limit its value. Not how the arrow is shown in the middle positions indicating that it is normally open but closes the flow off when the pressure feed forces the arrow against the adjustable spring.
The top valve shows an external Y drain line but only has 2 ports.
The lower symbol has a third tank line port and arrow heads pointing in both directions. In this valve, if the pressure downstream rises above the reducing valve's setting then the valve will vent this pressure down the third line to provide a constant pressure in the sensed port.
Learn more about pressure reducing valves