Technically, the content on any campaign you have created are gotten from the category you have chosen when you were creating such campaign. However, the category is a section of many sections in your library.
For example, the “inspirational quotes” category is a category under your library that contain posts relating to inspirational quotes only. If you decide to create a campaign about inspirational quotes, you must create content about inspirational quotes and add them to the “Inspirational Quotes” category under your library. Then you can go further to create a campaign about inspirational quotes by picking “Inspirational Quotes” as your category.