Well,
When a drawing is Xref'ed to an other drawing, you can use the xclip command to "erase" a part of the file. It acts sort of like trim or a cookie cutter.
what ever is out side the boundry is "erased" more less hidden on that current file.
nothing is actually erased out of the file it is hidden but not gone you can get it back by using xclip and again selecting the delete option.
DO I WIN!
