WordPress categories are a very convenient way to organize posts. Parent and child categories making hierarchical arrangement of posts possible. In addition, one post can have more than one category. This provides flexibility to show particular posts in widgets and menus.
Go to Posts -> Categories.
Add the category name and slug (will show in the category URLs) and select Add New Category.
Create a sub-category.
Assign Posts to Categories
There are two ways to add posts to categories. The first one is from within the post. Open or write a new post and select the appropriate category.
The second way is to bulk add posts to a category. To do this go to Posts > All Posts.
Select the posts to be updated.
Select Bulk Actions > Edit and then Apply.
Select the appropriate category and Update.
Now, the posts are assigned to the selected category.