C | Background Color (Colour) | The background color if an image is not used.
Img | Image (Bitmap / CubeMap) | A Bitmap, CubeMap, or file path to an image.
B | Is Background (Bool) | If true the image will be used as the background.
E | Is Environment (Bool) | If true the image will be used as the reflection environment for physical and standard materials.
L | Is Illumination (Bool) | If true the image will be used as a light probe illuminating the scene. (Only work with CubeMaps.)