Once you have created a subcategory, it will be shown nested within the parent category in the Categories configuration page. Subcategories can be used just like a normal category.
NOTE: Parent category labels do not appear alongside subcategory labels in the Agent interface. For example, you create a Customer questions category. Messages or threads assigned to that category will then be labeled Customer questions. But if you create a subcategory within Customer questions called Pricing question, only the label Pricing question will appear on messages or threads assigned to that subcategory. If they have the permission, agents can click on Recategorize thread at the top of the thread or Recategorize in the More menu of a message to see the parent category.