Working with DGN Files
A DGN file is composed of one or more models, each of which is represented using elements such as lines, arcs, and shapes. The File tab has items for opening and closing DGN files. These file management operations and others can also be performed from the Work page or File Open dialog.
You can have only one DGN file open at a time. If you open a DGN file when one is already open, the first file automatically closes.
You can, however, view the models contained in other attached DGN files.