The linear polarizing filter sheet is used to linear polarize a lightsource or camera. You can cut the sheet to make the filter fit in your application.
When to use a Polarizing Filter?
A linear polarizing filter is used to remove reflections from light sources on shiny objects. When a linear polarized light source hit a surface it gets again polarized in all directions, except the direct reflection. The direct reflection will stay in its original polarized direction. Mounting on the lens of the camera a linear polarized filter, that is polarized in a different direction than the direct reflection of the light source, the direct reflection will be blocked. As a result, the image will have no reflection of the light source.