- Fan On/Auto: This option relates to the indoor fan operation only. The Fan Auto selection will circulate indoor air only during a heating or cooling cycle whereas the Fan On selection will circulate indoor air indefinitely.

- Mode: This function categorizes your system's features and allows the user to select which feature to operate be it Cool for air conditioning, Heat for warming up the house, Off for the days of window weather, or Auto for an automatic changeover between Heat and Cool for the hot days with cooler nights (if your thermostat applies).

- Temperature Up/Down (+/-): This selects the desired indoor temperature target and does not determine output air temperature.

- Hold/Run Schedule: This feature applies only to programmable thermostats and gives the user the option to operate the central air systems by running a predetermined schedule or by holding a manually set temperature target. Some thermostats allow for timers on the Hold feature while others remain indefinitely until manually changed back to Run.

- Auxillery/Emergency Heat (AUX/EMG Heat): This feature typically applies to heat pump (electric heat) systems that use a secondary source of heat via electric resistance to support the primary source (heat pump) for high demand or replace it in the event that the primary heat source fails. Some systems use electric heat as the primary source with a gas furnace as the secondary. Whichever way your system is designed, setting your thermostat to this function will bypass the primary source of heat and run the secondary only.