ND64: The ND64 filter reduces 6-stops of light. The ND64 filter could still be used for video if shooting wide open (f2.8) on a bright sunny day. The other use of this filter is to reduce 5-stops of light for longer exposure photography.
ND128: The ND128 filter reduces 7-stops of light. This filter will be exclusively for shooting photos. You would never use this for video. The ND128 filter is going significantly reduce the light to the sensor allowing you to shoot creative long exposures during the day.
ND256: The ND256 filter reduces light by 8-stops. This filter is only for shooting long exposure photos. This is the darkest of the Exposure collection and will unlock the lowest shutter speed.