Before making some changes onto your design file, you might need to set a Mark at particular point in time, in order to undo the changes back to that point, later on.
Sometimes after setting the Mark and modifying the file, the icon "Undo To Mark" does not appear.
To make it reappear , toggle on the "Undo To Mark" setting as per image
Alternatively, use the key-in: Undo Mark
Set a bookmark in MicroStation V8i