Extra fields are field that you can use to put additional information to an article. They have types like date, link, text combo box for easy completion by the creator of the article.
For example: if you want your authors when they submit an article (car brand) to select between 3 options for additional info, you create a band extra field with predefined values of cars(BMW, Mercendes etc) then the author can select one of these values intead of writing the value. This prevents mistakes in typing.
You can assign an extra field through extra field group to a category and this is the only way to use an extra field to an item. Through its category.
