When you use a level as an underlay in the current view and use the Linework tool on the edges of elements from that level, the elements become part of the current view, and you can modify them.
For example, in a Level 1 floor plan view, you can specify Level 2 as an underlay. If Level 2 has a roof, you can select the roof in the plan view and then modify it. You can even underlay the same level as the view to select a ceiling or beams or other elements that are not shown in the view. You can use the Overhead line style for tracing underlays or define your own line style.