These settings provide control over the background and atmosphere (fog or atmosphere_volume).
There are two types of atmosphere in Arnold, fog, and atmosphere_volume. Fog simulates the effect of light scattering, which causes more distant objects to appear lower in contrast, especially in outdoor environments. Atmosphere_volume simulates light scattered by a thin, uniform atmosphere. It produces shafts of light and volumetric shadows cast from geometric objects.
You can choose None (no atmospheric effects), fog, or atmosphere_volume. If you select fog or atmosphere_volume, the Attribute Editor will display the controls for each node.
The aiFog and aiAtmosphereVolume nodes can also be found in the Outliner by switching to 'Show Dag Objects Only':
fog and atmosphere_volume selected in the Outliner (Show Dag Objects Only).
Use this setting to create a Background shader. Click and hold on the create button to create an Environment shader.
Background is considered deprecated and will be removed in a future release.