From In-Portal User Manual

Categories

This tab manages the categories to which the product 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 product. To designate the primary category, 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 product will appear in all of the categories listed on this tab, in the Administrative Console as well as on the front-end.