Attached xrefs can be nested: that is, you can attach an xref that contains another xref. You can attach as many copies of an xref as you want, and each copy can have a different position, scale, and rotation.
You can also overlay an xref on your drawing. Unlike an attached xref, an overlaid xref is not included when the drawing is itself attached or overlaid as an xref to another drawing. Overlaid xrefs are designed for data sharing in a network environment. By overlaying an xref, you can see how your drawing relates to the drawings of other groups without changing your drawing by attaching an xref.
Example: Attaching and Overlaying External References