DGN underlays can be detached from a drawing file.
You can detach DGN underlays that are no longer needed in a drawing. When you detach a DGN underlay, all instances of the underlay are removed from the drawing, the underlay definition is purged, and the linking path to the DGN file is removed.
To hide the display of a DGN underlay temporarily, you can unload it rather than detach it. This action preserves the DGN underlay and its location for reloading later.