Categories are a hierarchical tool for grouping articles on the platform.

Categories allow you to set an Access Level that restricts access to any articles that are in a specific category. For example, if you publish a group of financial reports you might want to restrict access to these assets. To do this, you might create a category called Financial Reports and assign the Access level Financial Reports to the category. (The Access Level Financial Reports must already exist as a Viewing Access Level.)  Assign all financial reports articles to the Financial Reports category, and those articles will be protected from general consumption. 

Categories are also useful for creating blogs. Blogs allows the site to display lists or groups of articles based on a category. In the manufacturing world, a blog to display results of line inspections could be useful.  Create a category Line Inspections, create an article for each time a line inspection occurs, and assign the Line Inspections category to the articles. Create a menu item of type Articles->Category Blog and choose the new category Line Inspections.  Menu items require an Access Level be assigned: be sure to set it appropriately.

Note: The use of Categories is optional.