This is our top-level category; looking at it another way, it is the root category.
Ideally, every other category should be a subcategory of at least one other, and every article should be in at least one category; many will fit well into two or more (e.g. location, year, and subject-matter).
A full list of existing categories can be found at Special:Categories.
This category has the following 3 subcategories, out of 3 total.