add-diff-file.
The diff file was typically created with the save-diff-file command, or by a saved configuration. The file can be made writable instead of read-only using the -rw flag. The clear-files command should be used instead of the deprecated -replace flag.
If any unsaved changes are found in the image object, the command will fail. Such changes can be discarded by using the -force flag, or first saved using the save-diff-file command.