A derivative work is clearly dependent on another work or image, but within that categorization there is a great deal of latitude, with things like vector trace and colorized on one end, and on the other posts like post #4336806. Despite being derivative, a derivative work can still be considered a distinct "work". Conversely, a third-party_edit is considered a mere modification of a work.
While doing artist research on post #4644800, I noticed that the post at yandere #817908 had been flagged for deletion for "Mirror image copy". Danbooru's Find similar function didn't return any hits, but looking into it further turned up post #3931971. With that in mind, should #4644800 be tagged as a derivative work or as a third-party edit? Should an artist tag for the pixiv account that posted pixiv #91307377 be created and applied to #4644800, or should the artist be sakura honoka (srhk0623) alone?