What is custom set of IPTC categoriesin Universal Thesaurus

[philburton]

Does a "custom IPTC category" have a special meaning?

Phil

[Mario]

In relation to what?
Custom IPTC categories were sometimes used in legacy IPTC decades ago, when I recall correctly.

If your images contain custom categories, the Metadata Panel will show them and you can work with them in the thesaurus. But it is not recommended for new files to use legacy IPTC data. IPTC has replaced the old format with XMP-based IPTR almsot 20 years ago. IMatch supports IPTCCore and IPTCExt fully of course.