To see what a DForm slider does, create a new Sphere primitive, select it and create a new DForm. Scale the DForm’s filed to be smaller than the sphere and move it up so it’s around the pole of the sphere. Now drag the actual DForm part up to pull the top of the sphere out. If you select the sphere you will see, in the Parameters pane, that it has a DForm group - go to that and adjust the slider, watch how it affects the distortion of the sphere.
Victoria 4 has several built-in magnets (DForms) that adjust her shape as you pose. When you fit a hair or clothing model to her in DS the DFOrms get applied to the fitted item too, so that it doesn’t get left behind when the effect kicks in on V4, and that’s why you have the slider on the hair, I think.