From In-Portal User Manual

This tab manages the categories of the article. The list shows all categories to which this article belongs. The administrator can add and remove additional categories on this list. At least one category must be listed, and exactly one category must be designated as the primary. The primary category determines the permissions for the article. To designate the primary category of the article, select the category and click on the Primary Category icon in the toolbar. The ‘primary' marker will move from the previous Primary Category to the one you have selected.

The article will appear in all of the categories it belongs to, in the Administrative Console as well as on the front-end. All of the article's attributes (such as the Category Lead Story) will be applied in all categories as well.