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I haven't been editing actively lately, but I'm a long-term watcher of these categories, and I've been noticing that you have been adding them incorrectly to some pages. There was a lot of prior discussion about the proper and improper use of these categories: please see Wikipedia:Categories for discussion/Log/2017 April 4#Category:Animal rights advocates. And the category page for Category:Animal rights activists explains what each of the categories is for. Relatively few persons will be in more than one of these categories.
What got these categories into their present organization grows out of the idea of WP:Defining: that "not everything a celebrity does after becoming famous warrants categorization". There has long been a problem of editors putting famous people (mostly entertainers) into the animal rights activists category just because the person has expressed support for animal rights, and the consensus has been that this should not be done. Ingrid Newkirk is an animal rights activist for our purposes, but Ron Jeremy is not (although he once was in that category!). Some advocacy organizations tout their celebrity endorsers, but that does not warrant categorization here unless that celebrity (Bill Maher for example) has long made animal rights advocacy a prominent feature of their public work.
That was a roundabout way of getting to the answer to what you actually asked. This category is for people who worked to advance animal welfare (in the trenches, as it were). Category:Animal rights scholars is for people who wrote books or developed conceptual ideas. And the difference between Category:Animal rights activists and Category:Animal welfare workers is that the former is specifically about people who were activists (as in direct action) or public advocates for the idea that animals intrinsically have rights, whereas the latter is about people who did work on behalf of taking good care of animals, as opposed to advancing rights for those animals (the difference between welfare in the sense of well-being, versus fundamental rights). So people from PETA or the Animal Liberation Front are animal rights activists, whereas people who set up animal shelters or who argued in a more general way for treating animals kindly would be classified as animal welfare workers.