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 18/20/33 represent.

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

[![enter image description here][1]][1]

Thanks


  [1]: https://i.sstatic.net/WNch1.png