I believe that it is more appropriate for me to simply include this in the Manual of Style, but as this is pretty big and I am aware that some people do not read the Manual (which, if you haven't I suggest you do. We promise it contains much, much more essential and important information that any Terms & Conditions article). So this would be a much easier way for me to connect with the community.

I've been noticing this for quite some time... Misconceptions about Wiki categories. Below are some common mistakes:

Subcategories' Categories

Cat example

Many categories do have subcategories, for example, Gallery has Island Gallery and Dragon Species Gallery. Do note that ALL'pages in the subcategory should fit into the larger category's requirement or criteria too. This is very prominent in the Dragon categories. So, as long as a single page in the subcategory cannot fit in the main category, the category should not be added to the subcategory. I see a lot of subcategories which only have one or two pages fitting into category X, yet, X is added to the entire category.

For example, on the right, you see a screenshot from the Category, Dragons: Rise of Berk. You see that Video game Dragons is one of the sub categories. This should not be the case, as Video Game Dragon not part of Dragons: Rise of Berk, but Rise of Berk is part of Video Game Dragons. Rise of Berk should be the latter' subcategory, not the other way around.

In the first place, this should not happen as all subcategories should have a clear, logical connection to the larger category. And you should always build up to bigger, broader categories. Let's say, from Exotic Dragons, to the larger categories Dragons: Rise of Berk dragons, Dragons: Rise of Berk, Category:Dragons: Rise of Berk Characters, Dragon characters from the Franchise, Dragons Characters, Franchise Dragons and Dragons. DROB dragons and DROB characters should both also be subcategories of DROB and the other categories after it. DROB should be a subcategory of Dragons from the Franchise and the others after it. And so on, ending with them all being subcategories of Dragons.

It may seem overwhelming, so here's a tip: Get to know as many categories and their criteria/requirements, and understand exactly what the articles and categories you want to add are about. Follow a systematic flow from a more specialised category to broader ones.

Book vs Franchise

This one is much easier to grasp. Anyways, I've seen a couple of categories that tend to overlap Book and Dragon articles, though specified (not explicitly) in the respective categories. One prime example would be Sea Dragons vs Tidal Class. I've seen plenty of articles being mixed up here and there, and occasionally, new ones sprouting.

I know that it can be hard to avoid, especially if you are unfamiliar with the books or franchise. Fans, even the most hardcore of them, don't know everything. So, to avoid this, tip: Familiarise yourself with the book or franchise counterparts. You don't have to binge watch or read the entire series, but skim through the existing wiki articles we have. They are quite short and concise, but cover the main points. And if you do chance upon similar yet clearly different categories, like those aforementioned, do ask around.

File Categories

Categories meant for files and media should not be used sparingly. Many categories have bits of media in it. These should only be included in categories designated for it, let's say, Dragon Roars, for Roar audio files, or Images from A View to a Skrill, Part 2, this one's pretty self-explanatory.

Using Defunct Categories

Many users keep adding on to defunctional categories, like the once existent Category:Tide class instead of Tidal Class. A few defunct categories still exist, many of which are repeated, like Alpha Class, Alpha Species and Alpha Dragons. As categories, unlike articles, cannot be renamed, a created category stays. As such, several repeated yet unnecessary or unwanted categories stick around. And as a community, we can't reach a consensus immediately.

What you can do is to alert admins or moderators of any repeated or incorrect categories for swift deletion. Or if their differences (like the Alpha one) remains ambiguous, do ask, so that we can tell you which article goes where.

I hope this helps. While some are just reminders, others are set in stone--made official in the Manual of Style. So do visit that page.