If you add a category before you add questions to the question bank, all questions created can be added to this category and they will then belong to the topic, rather than the quiz. This approach will make management easier when you are creating new quizzes using questions from previous quizzes.
Categories can be created within an existing category (ie a sub-category).
Go to Administration > Question bank > Categories
Scroll down the screen (categories may already be listed, that you can edit) you will see the option Add category
Choose a Parent category from the pull-down list (defaulted to your topic name) – you are most likely to leave it at your topic name, but you could also add a sub-category within an already created category
Give the category a Name
Add Category info (optional)
Go to Administration > Question bank – your category should be listed under the relevant Parent category
Once created, you can also Edit categories (delete, move etc)