Used for mirrors. An example can be found in the SDK Prefabs.

Disable Pixel LightsDisables real-time pixel shaded point and spot lighting. Pixel shaded lights will fall-back to vertex lighting when this is enabled.
Turn Off Mirror OcclusionDisables occlusion culling on the mirror. Enable this if you see objects flickering in the mirror.
Reflect LayersOnly objects on the selected layers will be rendered in the mirror. Objects on the Water layer are never rendered in mirrors.
Mirror ResolutionRendering resolution of the mirror (per eye in VR). Auto renders at the same resolution as the user's HMD or monitor up to the maximum of 2048x2048.
Maximum AntialiasingThe maximum level of MSAA applied to the image rendered in the mirror. Can be overruled by client graphics settings.
Custom ShaderThe mirror will use this shader instead of the default shader if one is provided.
Camera Clear FlagsSpecifies the CameraClearFlags that the mirror will use to clear the background before rendering. The default "From Reference Camera" will use the same flags as the camera rendering the mirror plane.
Custom SkyboxIf "Camera Clear Flags" is set to "Custom Skybox" this skybox will be shown in the mirror. If "Custom Skybox" mode is selected but nothing is provided the background will be black.
Custom Clear ColorIf "Camera Clear Flags" is set to "Solid Color" this color will be used as the background. Note that the alpha channel will be respected, so you can use this to clear alpha and use it in a custom shader (e.g. for cutout-style mirrors).