These curves visualize the mapping for non sensor-clipped pixels. Filmic let you define the upper (and lower) bound of the curve, by white (black) relative exposure slider, and various other parameters, and its possible to create asymmetric curves. Sigmoid let you define something like „the center point“ of a symmetric curve by the skew slider, if I understand that correctly…
Sensor-clipping is a different beast to handle, as the composition of the light spectrum was lost during capture and needs to be reconstructed from valid information of other channels and/or surrounding pixels.