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.
Let’s take a look at how to specify the attachment method and then the impact of specifying the methods. That will help you decide which to use for your specific situation.
Process of specifying the method type
AutoCAD-Based Products
The terminology is a bit confusing in AutoCAD-based products since you use the Attach command regardless of whether you will use the Attachment or the Overlay method of “attaching” a file. You can type the Attach command at the AutoCAD command line or select the Attach commmand from the Reference Panel on the Insert tab of the ribbon. Within the Attach command, you can “attach” a file using the “Attachment” method or the “Overlay” method.
When attaching an external reference (xref) in AutoCAD, the Reference Type option can be selected in the Attach External Reference dialog box after selecting the file that will be attached. Note that the default setting is the Attachment type, but if you set it to Overlay, then the next file you “attach” will use the Overlay option.
After a file has been referenced into the current file (called the Host file), the file can be changed between the Attach and Overlay reference types via the External References palette. This palette is opened by selecting the arrow at the lower right corner of the the Reference panel on the Insert tab of the ribbon. (Since this is a palette, it can be left open or allowed to Auto-Hide so it is readily accessible.) You can also open the External References palette by typing XR or by highlighting the xref in the drawing, right-clicking, and selecting the External References option from the context-sensitive menu that pops up at your cursor.
In the External References palette, highight the desired referenced file in the File References section, then go to the Details section and modify the Type to be either Attach or Overlay. (Note that when specifying the reference type during the initial process, the reference type is called “Attachment”, but when changing it later in the External References palette it is called “Attach” type.)
The Attach or Overlay specification method is consistent for other types of files, such as images and PDFs.
Revit Products
In Revit, files are linked into the host file via the Link panel on the Insert tab of the ribbon. When files are initially linked into the host file, they are linked using the Overlay reference type. The user is not provided with an option for specifying the reference type during the link process, so if the user desires the Attachment method, it must be done after the linking process. After the file has been linked, select the Manage Links function from the Link panel on the Insert tab of the ribbon. In the Manage Links dialog box, go to the Revit tab and the desired linked file and change the Reference Type as shown.
Impact of specifying the method types
The purpose of choosing either the Attach or Overlay method is to determine how the linked file will be seen in subsequent downstream files. To make a simplistic illustation, let’s say that you have 3 files with which you are working. You link (or xref) file “A” into file “B”. You then link file “B” into file “C”.
Using the Attachment method, if you link (or xref) file “A” into file “B”, then link file “B” into file “C”, you will be able to see all of the information in all three files when you are in file “C”.
Using the Overlay method, if you link (or xref) file “A” into file “B”, then link file “B” into file “C”, you will only see the information that is contained within files “B” and “C” when you are in file “C”. You will not see any of the information from file “A”. Only the information from the last file linked (or xreferenced) directly into the host file will be seen in the host file.
A distinct advantage of using the Overlay method is that you do not create circular references. A circular reference is when you reference (or link) file “A” into file “B”, and then reference (or link) file “B” back into file “A”. This circular references can cause problems in many different ways as it creates a looping situation. In some cases, this will cause the software to crash with a good possibility of corrupting both files that are part of the loop.
Examples of when to specify each method
A common situation in AutoCAD in architectural situations is to xreference the first floor plan into the second floor plan to make sure that walls line up. It is not uncommon for the second floor plan to then also be xreferenced into the first floor plan. In this situation, the Overlay method should be used to avoid circular references.
Apartment buildings, hotels, etc that have repetitious layouts can take advantage of having each repeating space (such as an apartment or hotel room) xreferenced into an assembled complete floor plan. In this situation, the Attachment method is better so that this assembly file can be xreferenced into other plans, such as an electrical plan. This allows for all of the xreferenced apartments (or rooms) to automatically show up in any files in which the assembly file is xreferenced.
In AutoCAD, it is common to xreference the floor plan(s) into elevation drawing files in order to properly locate walls, windows, doors, etc. In these situations, the Overlay method should be utilized. This eliminates the floor plan(s) from accidently showing up on files where it is not desired.
In Revit, you typically link the MEP file into your Architectural file. When you send your Architectural file back to the MEP engineer to link into their MEP file, you do not want a circular reference. Therefore, you should use the Overlay option.
The 2013 versions of AutoCAD and Revit were used in the article, but the Attach versus Overlay functionality works the same for previous releases.
Applying Technology to Architecture 