Bind External References to Drawings
External references (XREF) are links to an externally referenced file and not part of the actual drawing. You have to include all the external reference files, if you want to give a copy of a drawing, containing external references, to someone else. And on the receiving side the paths you used when linking the external references must be re-created or changed.
To make the process of sending a copy of a drawing with external references easier, you can first bind the external references to the drawing, which makes the external references a permanent part of the drawing and similar to inserting a separate drawing as a block.