That selector controls the mode doors that direct the air through the different ducts... You have floor vents, panel vents on the dash, and defrost vents on the dash... Newer cars also have demist ducts that hit the side door windows to clear any fog from obstructing the side view mirrors... The slider is configured like that so it changes the mode as you sweep through the travel of the lever from top to bottom or vice versa...
A/C typically is used with panel vents or floor vents, or a combination of both called bi-level...
Heat is used for floor vents and defrost and sometimes a combination of both...
The top lever controls the temperature door that switches the air between the heater core and the evaporator, depending on if you are using heat or A/C...
