If you want to know about differential thermostats, what they are, and what they are used for, take a look at this another page of mine: Differential thermostat
The differential thermostat is strictly necessary in a solar heating unit.
Obviously it will not be necessary in a thermosiphon type solar heater.
The differential thermostat is what manages the pump according to the data received by the temperature probes.
So there is not much need to instruct too much about its installation, especially when there is already documentation on the subject in the thermostat section.
Here I indicate the order of connections required by the differential thermostat:
The graphical representation of the parts that connect to the solar thermostat: