V-Ray Sky is special feature provided by the V-Ray renderer to work with the V-Ray Sun. Together, V-Ray Sun and V-Ray Sky reproduce the real-life Sun and Sky environment of the Earth. Both are coded so that they change their appearance depending on the direction of the V-Ray Sun.
When the V-Ray Sun and Sky systems are connected together, they will share a number of parameters. For example, changes made to the Sun section of the Sky Texture will directly affect the V-Ray Sun. See the V-Ray Sun and V-Ray Sky | SunLight page under Lights for additional information and examples.
V-Ray Sky texture cannot be created in V-Ray for Revit yet.