ConstructSim Planner uses the concept of overlay files. An overlay file is a V8 DGN file that references the original DGN or DWG file. It works as a container for review-specific content such as markup elements and saved views, while protecting the original file from modification.

When you open an ordinary (non-overlay) DGN or DWG file, ConstructSim Planner creates and opens a temporary overlay DGN file. If you modify this overlay file and try to close or switch to another file, you are prompted to save the changes to a permanent overlay file. However, when you close the overlay file without doing any modifications, it is deleted.

ConstructSim Planner opens existing overlay files directly. It does not create an overlay over an overlay.

Existing RDL or _redline.dwg files are treated like any other DGN/DWG files; an overlay is created for them.

When you open an ordinary DGN or DWG file, ConstructSim Planner does not automatically search for a corresponding overlay file. For example, say a folder contains both test.dgn and test.overlay.dgn. If you open test.dgn, ConstructSim Planner creates a temporary overlay file referencing test.dgn, ignoring the existing test.overlay.dgn. To open the existing test.overlay.dgn, you must explicitly select it.