The Planar Reflection Caster component is the script that takes a reflection and assigns it to a material. It must always be attached to a Mesh Renderer component and it will automatically track all of its materials and enable per-material settings to define whether the material uses the Reflection Color texture (_ReflectionTex), the Reflection Depth texture (_ReflectionDepth) and whether an additional Blur Pass should be used to blur the final output of the reflection.
In version 3.x of the asset this blur pass was used to simulate the roughness of the material and how it affects the sharpness of the reflection but in version 4.x all PBR shaders have been rewritten in all pipelines to use a much more accurate and performant blur method. Because of this, we recommend you to disable the blur pass in most cases, it is provided as a form of backwards compatibility with shaders from previous versions.