Joomla! Documentation™

For example, you might have a Category called "Pets" and in that category, subcategories called "Dogs" and "Cats". Articles about dogs would be assigned to the "Dogs" Category, ones about cats to the "Cats" 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.