Underlays are helpful when you need to understand the relationship of components at different levels for coordination and construction.
Video: Creating an UnderlayFor example, you may want to use an underlay to make it easier to place light fixtures above furniture in an architectural model. Underlays are also useful for working with MEP models, since you can work in the floor plan while viewing the ceiling plan as an underlay.
When you set an underlay, Revit displays another level of the model at the view's cut plane under the current plan view. The underlay can be viewed from above or below the current level. The underlay appears in halftone and is visible in a hidden line view.
When you create a new plan view, the Range: Base Level is set to None.