The way they usually work: You have numbers ranging from high to low. These indicate the % of humidity you are trying to maintain. 90 would bring on A/C at 90% humidity and 20 for 20% humidity. You can set it for max (90% or above) if you want to control cooling by temperature as opposed humidity level. We would tell people to leave it on 65-70% when they were gone for the year with the Thermostat set for 80 degrees, and "off" when they are there (the ones we sold had an "off" setting). Hope that helped!
Eventually, you may want to replace that with a programmable thermostat. Humidistats become less acurate with age!