V-Ray SphereFade Overview
VRay Sphere Fade is a volumetric shader which confines the rendering to only a portion of the scene (defined by an implicit sphere). Different passes can have different sphere fades (or not have one at all), thus speeding up rendering if parts of the scene are not required in a particular situation.
To create a VRay Sphere Fade node go to Pass Shaders->Volume->Add ->VRay_Sphere_Fade.
V-Ray SphereFade Parameters
Enable - enables the VRaySphereFade volume.
Empty color - specifies the background color which will be rendered outside the VRaySphereFade.
Affect alpha - when enabled V-Ray will render an alpha channel where everything outside the volumetric gizmos will be transparent and the objects inside the gizmos will have their normal alpha.
Falloff - a falloff radius that is used to blend the content of the VRaySphereFade gizmo with the Empty Color.
Gizmos list
Gizmo_objects - the actual gizmo objects that are going to be affected by the VRaySphereFade volumetric. Only implicit spheres can be used as VRaySphereFade gizmos.
Gizmo_invert - when enabled, reverses the logic of the sphere gizmos: everything outside the gizmos will be rendered.