Create an underlay when you need to understand the relationship of components at different levels for coordination and construction.
Use the Underlay parameters on the Properties palette to set an underlay. For example, you may want to set an underlay for the casework in the kitchen to make it easier to place the light fixtures above.
Range: Top Level is automatically set to one level above the Base Level. When there is an intermediate level, such as a stage, you may want to set Range: Top Level to the level above the stage.
The underlay displays in halftone in the plan view.
The following illustration represents the settings in the Properties palette where 1 = Range: Base Level, 2 = Range: Top Level, 3 = Look Up, 4 = Look Down