The Square Polarizer for the LEE SW150 System slots into the SW150 filter holder where it can be rotated to the desired angle.
If using with a ND Grad you should position the grad first and then slot in the Polarizer. Although you won't be able to separately rotate it you can flip it through 90° which can often help.
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.
It is a Circular Effect Polarizer (nothing to do with its shape!) which is ideal for most modern cameras because it doesn't interfere with their complex metering and auto-focus systems.