This message is sent back to the originator of a saved file if at some later time it becomes ‘safe’ (i.e. on disc).
You should compare the ‘your_ref’ in this message with the ‘my_ref’ stored when the original Message_DataSave message was sent. If the messages are the same then the file can be marked as safe, unless the user has edited it in the meantime.
Probably the best implementation is for your program to store the ‘my_ref’ with the file, but zero it on any modification, as 0 will not be a valid ‘your_ref’.
It appears that this message is designed for cases where one program saves a file to another program, and then that second program saves the unmodified file to disc.