The 4 x 4 Schneider Starter Kit consists of one high-quality Schneider 4x4-inch Neutral Density (ND) .9, Black Frost and Circular Tru-Polarizing filter, making a nice base kit of commonly used video filters. 
The Circular Tru-Polarizing filter is for cameras that use polarizing elements in their viewing systems' beam splitters, such as the ARRI 435 and the Movie-Cam Compact. To ensure correct placement on the lens barrel, Schneider clearly marks the outer surface on all Tru-Polarizing filters.