The NiSi ND64 (6 Stop) + CPL is a Neutral Density (ND) filter that uniformly reduces the amount of light entering the lens combined with a CPL.
ND Filters can enable greater motion blurring and image detail by allowing a large aperture and a slow shutter speed. ND filters are also helpful when the contrast between the highlights and shadows is too high to get a quality exposure. The combined CPL can reduce reflections and haze.
The ND64 6 stops ND filter is generally used during the golden hour to create exposures of 1-2 minutes for smooth water or clouds.
A 6 Stop is also great for capturing dynamic water while retaining some texture before the golden hour. Control reflections and the shutter speed in a single filter avoiding the need to stack circular filters.