Forum Categories also for products

It is only possible to use categories in the portal for the main helpdesk. For products it is not possible, because you have always to select a top level categorie in the settings for a product. Therefore the minimum level is at least THIS selected categorie. So on the portal itself, there can only be shown the "Forums" based on that categorie. In the main helpdesk, you can see all categories AND the respective forums. Maybe you can solve this by adding one more level over the categories or assigning categories to specific products.