At first glance the proportional valve symbol looks very similar to that of a directional valve. However, the symbol includes several key differences from that of the ON/OFF style directional valves. Firstly the outside of the spool section contains lines either side. The solenoids also have arrows running through them to show they operate gradually rather than just on and off.
The square box with the triangle inside represents internal electronics and the box with the G and upturned U represents closed loop control of a particular valve stage.
The top symbol shows a directly controlled proportional valve.
The second is a proportional directional control valve, pilot-operated, with closed-loop position control of the main and pilot stages, with integrated electronics.
The third is a servo-valve, pilot-operated, with closed-loop position control of the main and pilot stages, with integrated electronics.
The forth is a servo-valve, pilot-operated, pilot stage with electrical control mechanism with two coils, continuously controlled in both directions, with mechanical feedback of the valve-spool position to the pilot stage, with integrated electronics.
The bottom symbol shows a servo-valve with a preferred position in case of a power failure, electrical feedback and integral electronics.