This filter combines the 81EF with the linear polarizer, and is an essential outside all-purpose filter. It deepens skies and minimizes reflections, as well as warming the image. It is typically rotated to achieve the required effect; for this purpose you should ensure that your matte box has at least one rotating filter tray, and make sure that when purchasing circular filters that you buy a polarizer with a rotating ring.
For more information on polarizers, please see this
polarizers technical data PDF