The 105mm Landscape Polarizer is slimmer than the standard version and can be used on wide angle lenses without vignetting making it ideal for landscape photographers. It is designed for the LEE 100mm System.
In fact on a full frame sensor it can be used on lenses as wide as 16/17mm with a two slotted filter holder without fear of vignetting.
The Landscape Polarizer attaches to the filter holder via the 105mm Accessory Ring (sold separately) which fits to the front of the 100mm Filter Holder. This ring allows the Landscape Polarizer to be rotated independently of ND Grads or other filters in the holder - giving full creative freedom.
It is a Circular Effect Polarizer (nothing to do with its shape!) which is ideal for most cameras because it doesn't interfere with their complex metering and auto-focus systems.