I'm relatively new to hydraulics, and I'm trying to understand this schematic. I've got the majority of the system figured out, but I cant for the life of me understand what the symbols at positions 18/20/23 represent.

The unit actively controls a constant pressure in the two cylinders attached using an electronic pressure switch. Its the control unit of a hydraulic tensioning system in a ski lift. My feeling is that 18/20 may have something to do with electrical pressure cells?

Thanks