Subcategories give you the ability to further group related content within categories for better discoverability and organization. 

Subcategories are visible in the left sidebar or by clicking a category. They will always show above pages in your category sort order unless you've pinned pages or enabled custom order.

Related

Create and edit subcategories

Move subcategories

Add a new draft to a subcategory

Move an existing page to a subcategory

Delete a subcategory

Bulk move pages into a subcategory

Reorder, filter, and sort contents in a subcategory