A product can only be assigned to one category or sub-category. As you have the “Catalog Overview Mode” enabled, make sure when you add a product you also assign it to the respective category or sub-category so that once you click on the category, it will take you to the list of products assigned under that category or sub-category. That said, catalog overview mode doesn’t display custom fields. As an alternate solution, you can create custom fields or tags and then assign your products to more than one of these custom fields or tags. You can use the custom fields or tags to filter your products from the sidebar. However, keep in mind that custom fields and tags are not displayed under catalogue overview mode. We will discuss the viability of implementing something like this in the future.