6233638's points above are well taken. I can't speak to why it is the way it is, but I can tell you how I use it, and that might shed some light on some of the choices above (and why some of the proposed additions seem redundant).
I use the [Media Sub Type] field to categorize different kinds of a particular [Media Type]. By that, I mean: those which serve different fundamental purposes, or which have a set of characteristics that make them unique from "typical examples". As 6233638 points out, because you can "stack" different fields on top of the Media Sub Type for a particular file, you don't need the Media Sub Types to be very specific. But, a TV Show is fundamentally different from a Movie in many ways. You won't want to browse them using the same sub-categories.
Likewise, Audiobooks are fundamentally different from Music, such that it makes sense to treat them entirely differently.
However, for many items, you can easily "unify" the top-level sub type, and then further subcategorize them via another field. For me, the usual suspect is [Genre]. If I have a bunch of various "training" and "education" types of recordings, I can lump them all under the existing Educational [Media Sub Type], and then have a "Cooking" genre, a "Programming" genre, or whatever I want within there, to make those into sub-categories. But, with those types of things, I can pretty easily divide them into [Series] and even sometimes [Seasons], so the "category scheme" I'd want to use with them matches. So, I just need a "curated" Genre list, and I can keep a variety of Educational videos all in one place.
I can't keep Audiobooks and Music grouped together because they both use [Genre] for other purposes, so that makes sense to have as a distinct [Media Sub Type] (though I don't argue that a separate Book item isn't redundant, but again, "audiobooks" are a well-defined "category" in other applications so perhaps it makes sense in that regard).
Some of your other examples...
Music Videos could certainly be used for both MTV-style single-song produced videos, and for live concert video recordings (equally valid would be using Music as the [Media Sub Type] since [Media Type] is already video). But these are two fundamentally different things, and if you're looking for one, you're probably not looking for the other. And, Music Videos are a "thing".
What I do? I have my small collection of MTV-style music videos tagged as Music Video (small because YouTube makes it basically pointless to collect all but rare ones). I have my (also reasonably small) collection of Concert videos tagged as [Media Sub Type] Music. That distinguishes them. We don't have a Concert Media Sub Type, but that works basically just like one (but with a more vague name so you could jam whatever weird music video you have in there if needed). Each of those "types" is likely to want to use their own [Genre] set, since Genre is a thing in music.
So... I don't know.
I get these from the voting list above:
* Sports: Again, they're not really TV Shows (they're way longer than most, anyway, and usually not consumed in the same manner), and certainly not Movies. I don't really record a lot of sports stuff, so when I do, they just get put in as TV Shows. Sporting Event is the same thing with a different name, and you can use [Genre] to subcategorize them as needed anyway.
* Concert/Live Performance: This could almost be combined with Comedy, except that stand-up Comedy things are often (but not always) going to fall into Series categories, wheras for concerts and other live performances, you probably want [Artist]. But then there's Plays... So, I don't know. I like the Media Sub Type names to be very short-and-sweet, so I'd say "Live" in any case. I'm not sure if you really include Concert (which could be under Live and could be still under Music like I do now), but I think it is something that people are likely to collect, and which have a "defined set of traits" that makes them difficult to mix with other media (they have Genres, you browse mostly like Music, etc).
* Comedy: Because stand-up is different from a TV Show and also different from a Movie. You care about different categories and fields for these. I have these under Other then Genre = Comedy now, though, and that works okay for my needs. I don't think having separate audio and video comedy choices makes any sense, because we already have [Media Type]. Check that properly if you want them separated. It could conceivably fall under a more-generic Live Sub Type, along with Concerts and whatnot.
* Workout: I used to keep these under Educational (with cooking shows, my technical training videos, and stuff like that) but my wife could never find them, so now they're stuck in my TV Shows. She'd like a separate nicely-named category for them. And what she likes, I like.
Things I don't get:
* The Animated ones. That's a Genre, and it is even one that auto-populates for you from TMDB (and it is easy enough to set it with Tag On Import rules for other shows). Separate them using Genre if desired. I have views that show just kid-friendly shows and movies (and my other ones hide that content) and you don't need fancy Media Sub Types to do it, just filters on Views.
* The Comedy Audio and Video ones noted above.
* Training. That is Educational. Use it and add a Genre tier if needed.
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Topic: POLL: Proposed Media Sub Types (Read 4111 times)