Understanding External Reference:

External references (Xrefs) allow you to attach drawing files within your current drawing, enabling collaborative work and data sharing.

Xrefs can be attached as Overlay or Attachment. Overlays do not include nested Xrefs, while Attachments do.

Identifying the Problem:

The exhibit shows that the Buildings drawing is nested within the Contours Xref.

To remove the Buildings drawing from the C-3 drawing, it must be detached from the Xref hierarchy.

Steps to Detach Xref:

Right-click (Control-click) the Buildings reference in the Xref Manager.

Select the Detach option. This will remove the Buildings reference from the drawing.

Conclusion:

To remove the Buildings drawing from the C-3 drawing, detach the Buildings reference directly from the Xref Manager.

AutoCAD User Guide on Managing Xrefs.

AutoCAD Help Documentation on Detaching Xrefs.