If an insert (such as a window) and its host (such as a wall) do not have the same values for the Phase Created and Phase Demolished properties, Revit Structure automatically places an infill element in the host to patch the hole created by the demolished insert.
The infill element lets you place inserts in one phase, demolish them, and then place new inserts in the same location. The infill element assumes the same structure as its host. In certain cases, you can modify the structure of the infill element by changing its type.
Infill elements for roofs and floors project down from the top face, and infill elements for ceilings project up from the bottom face.
You cannot drag, move, mirror, rotate, copy, or paste an infill element.