The LEE 105mm Rotating Polarizer is the most popular Polarizer for the LEE 100mm System and is the recommended choice for most photographers.
The Polarizer removes light which has become polarized by bouncing off flat non-metallic surfaces such as glass or water. Filtering this light can eliminate reflections, remove glare, darken blue skies and generally increase saturation.
The Polarizer attaches via the 105mm Accessory Ring (sold separately) which fits to the front of the 100mm Filter Holder. This ring allows the 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.