Subcategories are secondary titles that follow the main title "Category". They don't exist unless the "Category" is there. They help you in organizing your store products.
How to add them?
1. Open your store account
2. From Left Hand Side Menu, navigate to Categories and click on it
3. Choose from the list you will have click on " Sub Categories"
4. A new interface will appear to you "Manage Sub Categories", click on one of the two add subcategories buttons to add one,
5. Fill the blanks with the required info about your new subcategory and link it to the main category
6. After you're done click on "save"