The DWG file format does not support all of the reference types available in MicroStation. As a result, you must use the Save As DWG/DXF Options dialog to define how the following types of references are merged into the model:

• external attachments — External attachments are separate files that can be attached to the main (default) design file as a reference. These attachments can be retained as separate attachments (and become AutoCAD XREFs), or be merged into the default model.
• self-attachments — Self-attachments are references to the main design file from within, such as in the case of reference views used to create sheet models. The DWG file format does not support self-attached references. They must be merged to be usable in AutoCAD.
• attachments to non-default models — The DWG file format only supports one design model. DGN files can have multiple design models. One of these models is defined as the default model. Attachments to non-default design models must be merged to display in AutoCAD.
• masked attachments — The DWG file format does not support clipping masks applied to references. Attachments with clipping masks must be merged, or the clipping mask will be removed in AutoCAD.

Additionally, in the Save As DWG/DXF Options dialog, you can enable/disable the Merge Displayed Levels Only setting to define whether or not only those levels displayed in the selected view are merged into the DWG file. If the configuration variable MS_MERGE_DISPLAYEDLEVELSONLY is set, then the Merge Displayed Levels Only setting is turned on and disabled.