External References (XREF) are one of the most important concepts to understand in a CAD environment. The idea is simple enough: link one file to another so that any changes made to the source file, will show up in the destination file as well. Every CAD tech. I know can explain this basic concept to me but still, I see Xrefs being ignored, or misused, on a regular basis. Let's get into details on exactly what Xrefs are and the best ways to use them to make your life much simpler.
OK, so what exactly is an Xref and why do you want to use one? Well, imagine that you have a set of 300 drawings and the title block calls out the number of files (i.e. 1 of 300, 2 of 300, etc.) If you have put your title block in every plan as simple text then when you add another drawing to your set, you'll need to open every single file and modify the sheet numbers one at a time. Think about that for a moment. You'll need to open a drawing, wait for it to load, zoom to the text you need to change, modify it, zoom back out, then save and close the file. How long does that take, maybe two minutes? Not that big of a deal for one file but if you need to do 300 of them, that's ten hours of time you'll spend just to change one piece of text.
An Xref is a graphic image of an external file that appears, and prints, inside your drawing just as if it were drawn inside that file. In this example, if you created a single title block and inserted the "graphic snapshot" of an Xref into each of those 300 plans, all you need do is update the original file and the xref in the other 299 drawings is immediately updated. That's two minutes versus ten hours of drafting time. That's huge savings.
How Xrefs Actually Work
Every drawing has two spaces that you can work in: model and layout space. Model space is where you draw items at their actual size and coordinate location, while layout space is the place where you size and arrange how your design will appear on a sheet of paper. It's important to know that whatever you draw in model space of your source file can be referenced into either model or layout space of your destination file but anything you draw in layout space cannot be referenced into any other file. Put simply: anything you want to reference needs to be created in model space, even if you plan to display it in layout space.
1. Create a new drawing (this is your source file)
2. Draw whatever items you want to reference in model space of the new file and save it
3. Open any other file (this is your destination file)
4. Execute the Xref command and browse to the location where you saved your source file
5. Insert the reference at a coordinate location of 0,0,0 (a common point to all files)
That's all there is to it. Everything you drew in the source, now displays in the destination file(s) and any change you make to the source drawing is automatically displayed in every file that references it.