The World Output node is used to output light a color information to the scene's World.
The appearance of the environment, usually preceded by a Background shader.
It is not possible to have an HDR and volumetric due to the fact that HDR's are assumed to be an infinite distance from the camera.
Render engine the input shaders are used for. By default shaders are shared between Cycles and Eevee, with multiple output nodes specialized shader setups can be created for each.
This node has no outputs.