B+W 4x4 Neutral Density Filters attenuate the light by 2, 3, 6, or 10 f-stops. This is beneficial to achieve the correct exposure when the brightness of the subject is still too high for the fastest shutter speed and the smallest aperture, or for that "cotton candy" look to streams and waterfalls.
ND.6 (802) = 2 stops compensation
ND.9 (803) = 3 stops compensation
ND1.8 (806) = 6 stops compensation
ND3.0 (810) = 10 stops compensation