From In-Portal User Manual

This tab manages the categories of the topic. The list shows all categories to which this topic 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 topic. To designate the primary category of the topic, 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 topic will appear in all of the categories it belongs to, in the Administrative Console as well as on the front-end.