"label", "tag" or "keyword".
In common use and in apps handling the topic.
Whereas "category" is used for the more abstract concept
like (semantic) groups of... well... labels.
I don't think it's a good idea to use a designation
that is used everywhere in a different meaning.
It established that for XnViewMP users, missleading them
and will lead to confusion.
Better use "label", "tag" or "keyword" instead of "category".
(Don't kill the messenger
)