Environment maps are images that wrap around your entire 3D world. A 2:1 ratio equirectangular image that is formatted to wrap seamlessly works best. Physical Evironments are used to light the models in the scene along with scene AE lights.
Control the bightness of the evironment map and reflections.
Control how much AE lights affect the scene. Useful if reflections are strong and are overpowering shadows.
Lets you toggle the Environment map on and off in your scene's background.
If you wanted to use a layer as the Environment map, you can choose a layer in your project from the Override Layer drop down to activate it. Another way to do this would be to use the Custom Texture Maps
Fine tune the X,Y, and Z rotation of your Environment.