Help balance exposure and/or compensate for the limited dynamic range of digital cameras. Add two stops of ND to control overexposure of the sky or exposure of a scene from side to side. For example, a scene looking down a city street with buildings on one side in direct sunlight and shaded buildings on the other. The same technique can be applied indoors when a window or doorway blows out. For more of a film look, the cinematographer can decrease depth of field by shooting wide open.
The ND combo combines a solid neutral density filter (0.3, 0.6, 0.9, 1.2) with a soft edge 0.6 ND. In all densities, the additional soft edge is two stops more than the lower half of the filter.