Whether linking files in Autodesk Revit or xreferencing files in AutoCAD, choosing to Attach or Overlay is an important decision. As I perform software training at companies, I routinely encounter many users who do not understand the difference between using the Attachment or the Overlay options. There is not a global right or wrong answer to this decision, but understanding the difference helps you to determine which option to use to achieve the results that you require. When you link or xref a file into your current file, you are doing that to see information that is in another file. The Attachment/Overlay option determines how that file will be seen downstream in other files.