overlay file

An overlay file is a type of multiple drawing. After a file has been loaded using the normal OPEN command, overlay drawings can loaded atop it. Overlays are completely separate drawings, and always maintain their separate nature. When an overlay is opened atop a separate file, it never integrates itself with another drawing. Rather, it shares the same coordinate space. Overlay drawings can be opened and edited independently. A reference file is simply an overlay that is read-only. A read-only file is one that cannot be edited.