A Section contains one or more categories, and each Category can have articles assigned to it.
One Article can only be in one Category and Section.
For example, you might have a Section called “Pets”, and categories called “Dogs” and “Cats”. Articles about dogs would be assigned to the “Dogs” Category, ones about cats to the “Cats” Category. So an Article about dogs would be in the “Pets” Section and the “Dogs” Category. You cannot have one Article that is in both the “Cats” and “Dogs” categories. To work around this, you could either
(1) create a new Category called “Cats and Dogs” or
(2) create a second Article to put in the second Category