Sometimes you need to refer to another drawing without inserting it. You may want to use part of another drawing as an example for your current drawing, or to see how the model in your drawing fits in with models in other drawings. You may want the flexibility of seeing the other drawing sometimes, but not seeing it at other times. Using an external reference is like laying one drawing on top of another and being able to see both at the same time. You can easily load and unload the other drawing.
External references, commonly called xrefs, enable you to view any drawing as a reference while in your current drawing. The external drawing is not part of your current drawing. The current drawing keeps track of the location and name of an external reference so that you can always reference it easily. As with blocks, you can snap to objects in the external reference, thereby using it as a reference for the drawing process. You can also change the visibility settings of the xref's layers.
Xrefs have several advantages over blocks:
Xrefs keep your drawing smaller than blocks. The externally referenced drawing doesn't become part of your drawing. Your drawing maintains only a reference (name and location) ...