Several people working on a project using some of the same xref bases..
So you need to add a layer to the xref base for your discipline or use....
And you don't want to screw everyone else up with thier particular drawings who are using this xref..
So what I do is create my layer in the xref base, draw my geometry on it (etc), FREEZE that new layer and save.
Then RELOAD the xref and thaw the new layer in my drawing, everyone else wouldn't even notice because I froze it before saving.