The function interface-document-modified-p
gets and sets the document-modified flag in the interface
interface
.
Currently this only has a visible effect on Cocoa, where an interface whose document is modified is flagged by adding a dark dot in the middle of its Close button (the red button at top-left of the window).
On other platforms the document-modified state is merely remembered.
CAPI User Guide and Reference Manual (Windows version) - 3 Aug 2017