Q&A Admins are able to create categories in Q&A. This in not mandatory.
Q&A categories can be used for filter and assign purposes.
When Q&A categories are created, Question Drafters and Question Submitters are able to select a Q&A category in the question dialog. Per question one category can be picked.
Q&A Admins are always able to amend or add the category when the wrong or no category is selected.
A common way to use the Q&A categories is to add categories named after the main subjects, usually the names of the main folders.