Selecting Topics (Subcategories)
Topics are a way to organize articles and have certain benefits such as being able to be featured on the homepage or in the sidebar with the Topic Widget.
Adding topics is easy however topics relies on Tags which means you must have articles on your site with tags defined in order to make some of those tags topics.
To add Topics click Settings > Tags
Then go to the tag that you'd like to "Mark as Topic." If you don't see your match in the list of tags, you'll first need to add it to an article then return to make it into a Topic.
Once a Topic is added you can go a step further by making it Featured on the homepage.