What are the qualifications for a special interest??

So I don’t know if I have special interests or if I just hyper fixate on certain topics, tv shows, or hobbies. Some of these fixations of mine only last a short while— for example; I got really into roller skating for like two months but don’t avidly keep up with it as much anymore. On the other hand, there’s been interests that outlast others and become almost obsessive (though I’m not sure what deems something obsessive versus being really invested. Any input on that would be helpful). One example is Sailor Moon which I’ve loved since childhood. I still actively watch it, purchase items pertaining to it, read the manga, and talk about it to my close ones regardless of whether or not they’re into the anime. There’s countless more examples but for each tv show or topic, I form categories in my mind and learn and indulge in them as much as I can. Bouncing off the Sailor Moon example, I know a lot of behind the scenes facts and even wrote a seven page paper on the history on the series. So my question I suppose is- am I just really into a fandom or is this a special interest? What counts as a special interest? Can you have multiple special interests? Do special interests fizzle out with time? What are your special interests?