@ Kirk,
I know the Categories / Supplememtal Categories fields are outdated, as said at the IPTC.org,
but still exists in them most software. Because custom fields are not supported in all software, I use these old ones for some extra infos e.g. photo genres, to structure my jobs a little bit more. eg. commercial, editorial, etc.
The Job ID / Transmission Reference field, I use for, well jobs;-) With the Job ID I can mimic (software independent) collections and albums. That's why I miss them in Photo Mechanic +