Create an underlay to a view to reference geometry from other plan views of the model.
This video demonstrates the following:
An underlay is helpful when you need to understand the relationship of components at different levels for coordination and construction. For example, you may want to use an underlay to make it easier to place light fixtures above furniture in an architectural model.
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. To set an underlay, you'll use these three parameters located in the Underlay group in the Properties palette.
In this example, we'll set an underlay for the casework in the kitchen to make it easier to place the light fixtures above. For Range: Base Level, select Level 1, which is the level for the casework. Notice that For Range: Base Level, select Level 1, in this example, Level 2. It's helpful to know the Range: Top Level, especially when it isn't the one you expected. For example, when there is an intermediate level for a mezzanine, you may want to set the Range: Top Level. The underlay displays in halftone in the plan view.
Next, set the Underlay Orientation. When Underlay Orientation is set to Look down, the underlay displays as if you are viewing it from above, like a plan view. When the Underlay Orientation is set to Look up, then the underlay displays as if you are viewing it from below, like a reflected ceiling plan.
New plan views will have their Range: Base Level set to None.
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.