In the listing of IPTC tags, there is an entry for Supplemental Categories with the tag SupplementalCategories. In Photo Mechanic 6 by CameraBits there are three supplemental categories in their IPTC Template. Although this may not be consistent with the IPTC standard, it is still a very useful feature. If one enters info into a sidecar xmp file with the exiftool command (exiftool -xmp:supplementalcategories="some text" file) the information is entered into the PM6 field Supplemental Category 1. There appears to be no way to enter data into Supplemental Category 2 or Supplemental Category 3. Can you add tags SupplementalCategory1, SupplementalCategory2 and SupplementalCategory3 (or some such) that allow entry into the three categories in PM?
You can add any non-standard tags you want as user-defined tags via the config file (https://exiftool.org/config.html)
- Phil