Running a wordpress action when a custom post type term (taxonomy category) is changed

‘set_object_terms’ is probably the hook you want. It gets passed the following: post_id, terms, term_taxonomy ids, taxonomy, append, old term_taxonomy ids.

You can use the post_id and taxonomy params to determine if its the post type and taxonomy you want to hook into, and then check if term_taxonomy_ids differs from the old term_taxonomy_ids to decide whether anything was changed.

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