This is water temps. Temp at the panel is an air temp or sunlight temp sensor. This tells the system either:
Yes, the sun is out, solar will warm the water if sent to the panels.
or
No, too much cloud cover, shade, or it is night time, keep the water off the roof, there will be no heat gain. That sensor has no influence over the differential.
This is taken from the documentation i have, so it may be a bit technical. Like i stated above, you'll probably be good if you leave it at the default setting(s).
Start: Sets the temperature diff to start heating from 3 to 9 deg. If set to 3 deg, this ensures that the temp has to deviate by 3 deg at least to the specifyed set point temp before it switches on. once on, it will start converging as it is heating. This is so it will not continually cycle.
Run: Sets the temp diff to stop heating from 2 to 5 deg. This setting sets how close to the target set point temp to switch the solar off.
See, if the (solar) system was set to go on and off with no differential, by the time the set point heated water or cold non heated water made it back to the sensor (at the equipment) to turn on/off the solar, the pool could gain/lose more heat, causing the final hot/cold temp of the pool water to be in excessively hi or low, of your set point.
The bigger the pool, the more you would need this differential option.
Does that make sense?