reference file

An overlay file is a type of multiple drawing. A reference file is simply an overlay that is read-only (cannot be edited). After a file has been loaded using the normal OPEN command, reference drawings can loaded atop it. Reference files are completely separate drawings, and always maintain their separate nature. When a reference 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.