Underlays

Underlays are essentially raster references. Following is the behavior of underlays in ProStructures.

DGN underlay in a DWG file - When you open a DGN underlay, it becomes a DGN reference attachment with the logical name "DGN Underlay" followed by a numerical index.
When you save a DWG file, a DGN reference attachment with logical name that has the prefix "DGN Underlay" becomes a DGN underlay.

You can also create DGN underlays when you Save As a DWG file. In the Save As DWG/DXF Options dialog References tab, select "DGN Underlay" for the External Attachment setting. All externally attached DGN files will become DGN underlays.
PDF underlay - A PDF underlay in a DWG file becomes a PDF raster attachment. It is supported to the extent of the raster manager's capability of attaching a PDF file.
DWF underlay - DWF underlays are not supported in ProStructures.

Note: In non-Bentley products, there are limitations to what you can do with DGN underlays. Hence, an xReference is recommended over a DGN underlay.