Downforce is a recent post on the forum. It gives a bit of insight as to how this works. Might help explain something, at least.

The basic concept is a front slider and a rear slider for aero inclination. This should be the angle of the front splitter/wing and rear splitter/wing. The above thread tries to answer how these interact with each other on the car, in a relatively simplistic perspective.
Then there is a Downforce slider, which controls how much effectiveness the pairing has on the overall car. The higher it is, the higher the downforce overall will become. Presumably, this effect is applied after the individual front/rear factors have been calculated.