So I'm the mod of game and I'm trying to have certain levels that are not bossfights in the Level category, but the boss fights are already in the levels, how do I add them separately? If any one has access to a tutorial of categories and subcategories, please link in comments! (Sorry if I described the issue badly)

Do you mean that you want a level category that appears on only certain levels? Rather than for all the levels.

Currently this isn't possible. And instead you'll have to use a level category specific subcategory variable instead.

Or you can create this level category, and for the levels where this boss fight doesn't exist, write in the category's rules that it doesn't exist and you won't be accepting submissions to this category. Or submit a run with a player called "N/A" with you explaining the same thing in the description.

Aw, alright. Thanks for explaining! If you can, can you elaborate what a level category specific subcategory variable is?

I think this is done for balance