This plugin adds radiobuttons near every category (even if you add a new category) so you can choose the primary category of your post **. Selecting a primary category and updating the post saves this data into a custom field with a meta key of ?primary_category?.
To display a list of all posts with a particular primary category, use the shortcode [primary-category name=?CATEGORY NAME? post_type=?POST TYPE?] where CATEGORY NAME is the name of the primary category you wish to display and POST TYPE is the custom post type name. If a name is not specified, the shortcode will default to all posts with ?uncategorized? set as the primary category. images or post.