In DWG workmode:

• When attaching a reference, you can specify a level for the reference attachment.

  In an AutoCAD DWG or DXF file, each XREF or layout viewport is placed on a specific layer. When you open the DWG file in MicroStation, XREFs and layout viewports are treated as references, and are placed on the same levels (layers). To modify a reference's level, open the References dialog, double-click on the reference to open the Attachment Properties dialog, and modify the Level setting.

  If a level is frozen, references on that level are not displayed.

• A model cannot be attached to itself as a reference (self-reference).
• Clipping masks cannot be used on references.
• You can work with live nested references, but cannot turn off live nesting or control nesting depth, except by using the Ignore Attachment when Live Nesting setting (Attachment Properties dialog.)