I have set the ReferenceNumber tag in all my image files, to give them a unique identifier despite renaming or moving within the directory tree. At the time I did not know that this tag is not supposed to be set unless the ReferenceSource tag is also set. I have had the ReferenceNumber unset is a few images, some of which, at least I have viewed, and viewed the IPTC tags, in IrfanView. Does IrfanView enforce the requirement for a ReferenceSource tag by deleting the ReferenceNumber tag if it is not present? If not, are there any other known processes which might do this?