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Turning on the Reference Display allows you to see objects on a floor below or above the one you are working on.
Saved Plan Views, Layers & Annotations
2421 - Saved Plan Views
4 - Understanding Layers
5483 - Understanding Layer Sets
240 - Understanding Default Sets
904 - Controlling What Displays in 3D
5435 - Using the Reference Display
5302 - Active Layer Display Options Side Window
362 - Showing an As-built Mask on a Remodel Plan