An xref drawing containing CADmep objects is treated the same way by AutoCAD as a drawing which does not contain CADmep objects.
AutoCAD treats an xref as a type of block definition with some important differences.
When you insert a drawing as a block reference, it is stored in the drawing and is not updated if the original drawing changes.
When you attach a drawing as an xref, you link that referenced drawing to the current drawing.
Any changes to the referenced drawing are displayed in the current drawing when the current drawing is opened. Xrefs must be model space objects, and they can be attached at any scale, location, and rotation.
A drawing can be attached as an xref to multiple drawings at the same time. Conversely, multiple drawings can be attached as external references to a single drawing.
Xrefs can be nested within other xrefs. The xref you attach to your drawing is included when your drawing is attached as an xref to another drawing.
Like a block reference, an xref appears in the current drawing as a single object. However, you cannot explode an xref without binding it first.
Annotation text sizes are taken from the original drawing (the xref) unless the styles are already present in the recipient drawing.
If the Xref is bound and then duct is exploded, the same duct information is available as if the original duct drawing was opened.
Binding an xref to a drawing makes the xref a permanent part of the drawing and no longer an externally referenced file.