Isn't that the same as layered swimsuit, should I alias it? Also would x_under_clothes and x_over_clothes qualify for layered clothing? (when x is a piece of clothing as well)
I want to say that layered clothing should only be for overlapping clothes of the same kind, since those seem to be the requirements for the specific tags, but that would mean layered clothing is a tag that has items that are too different as its implications, and hence it would be too broad and useless.
If we say layered clothing should only be for layered items of the same kind, then an umbrella is not very useful because there's nothing truly in common between some of these tags, other than the naming. A layered skirt is often just flaps (post #4592165), not two whole skirts one over the other, while layered bikini instead more often than not is two actual bikinis one over the other (post #4725527).
That argument aside, "layered clothing" makes me think of heavy winter clothing, or some weird fashion where mixed clothing is used, like coats over jackets or sweaters over dresses. And if you look at the actual posts under layered_clothing -layered_kimono you'll see a lot of results like that - things like bodysuits under jackets (post #4663170), multiple layers of different clothes like a normal school uniform (shirt, vest, jacket, post #4649949), coat over sweater over shirt (post #4421907), cardigan over shirt (post #4444139), etc.
It doesn't look like this tag is being used very consistently right now.
I see, I think I get what you are saying but still "Any clothing assembly that has multiple overlapping items." wiki explanation is still quite open-ended as well so anyone who would know that tag would use it in a broad sense and make a mess of it anyway so I think something here needs to be changed, either the implication or the wiki.
That argument aside, "layered clothing" makes me think of heavy winter clothing, or some weird fashion where mixed clothing is used, like coats over jackets or sweaters over dresses. And if you look at the actual posts under layered_clothing -layered_kimono you'll see a lot of results like that - things like bodysuits under jackets (post #4663170), multiple layers of different clothes like a normal school uniform (shirt, vest, jacket, post #4649949), coat over sweater over shirt (post #4421907), cardigan over shirt (post #4444139), etc.
This is a the more useful definition in my opinion, and could also be used for cases were there are prominent undershirts or an open shirt over another, which I think is definitely taggable and searchable. Kino (Kino no Tabi)'s full outfit fits this well too (trench coat over jacket over dress shirt; post #3466527). I think it would sense to repurpose / redefine layered clothing into this concept. I'm not sure if there is a more sensible tag name for that concept. From a brief scan it seems that's largely what the tag is being used for anyway.
For single garments, defining what a "part" is seems problematic. I mean maybe a dress over a petticoat should count, but those are arguably separate garments and fall under the above. For cases where there are stacked layers of cloth on the same garment, they are usually ruffled for volume and if so, would in most cases be covered by frilled_*.
I suppose there are things like the odd belt/skirt thing they have on the uniforms in Shingeki no Kyojin (e.g. post #1430378) that definitely wouldn't be covered by frilled_*, but I'm not certain that's not a separate part anyway (maybe a sash?), or if not that it's taggable in isolation anyway since it's such an oddball piece of clothing.
There's also Usada Pekora's outfit that seems like it should be "layered", but that's also separate garments intended to be worn together, (a leotard underneath a half dress/coat sort of thing).
Updated the wiki. Outside of Danbooru, layered clothing means an outfit with multiple layers, like a jacket over a sweater over a shirt, not things like a layered skirt, which is a single item of clothing.