Xrefs are the foundation of collaborative civil design. They allow you to link one drawing into another without permanently merging the geometry.
Often, you need to go the other way—sending a design to someone who doesn't have Civil 3D. If you simply send a Civil 3D file, they will see "Proxy Objects" (meaningless boxes). To "break" the link correctly for a 2D deliverable: Use the Export Civil 3D Drawing command. civil autocad 2d drawing link
In Civil 3D, type XREF . Select "Attach DWG" and choose your 2D file. Xrefs are the foundation of collaborative civil design