Categories and subcategories
When creating a new article from within a category, this category is automatically assigned to the article. After an article was saved, or even while creating it, you may change the category by opening the "Article settings" and selecting a different category.
In addition to the main category, you can also assign a subcategory to any article. To create a new subcategory, go to any of your main categories and open the "Category settings". Find the section "Subcategores" to create a new subcategory within a category.
Now go back to your article, open the "Article settings" (as seen above) and find the section "Subcategory".