What is the R-Slider Tab?

This is a built-in jumper switch in case there are two transformer systems. If there is only one R-Wire and it is connected to the R, Rc, or Rh terminal on the old thermostat, set the slider to the up position (1 wire). (Slider will automatically hide Rc position and internally jump R and Rc together.) If there is one wire connected to the R terminal and one wire connected to Rc terminal, set the Slider to the down position (2 wires).