The Lee SW150-6NDG-S 150 x 170mm 0.6 Graduated Neutral Density Filter is for the SW150 Filter Holder. For the landscape/outdoor shooter, the 2-stop Graduated Neutral Density Filter that comes with the basic SW150 system will more than get you in the game. The 2-stop filtration will allow you to blur that rippling water or make that high-flying bird seem to streak through air at super speed.
However, there may be times when you want or need to tweak your shot to a more precise degree. That extra half stop can make the difference between a good shot and a shot that "sings". The extra width of this filter allows for movement within the holder to adjust for the horizon line.
Determining which neutral density filter yields ideal results for any given lighting situation takes knowledge, experience and a collection of such filters.