This plugin was developed to 10up tests;
Many publishers use categories as a means to logically organize their content. However, many pieces of content have more than one category. Sometimes it’s useful to designate a primary category for posts (and custom post types). On the front-end, we need the ability to query for posts (and custom post types) based on their primary categories.
This plugin allows publishers to designate a primary category for posts, selecting it in a metabox on edit posts page.
To list the posts by primary category, use the shortcode:
[primary-category-posts category="test_category"]
Where test_category is the name of the category selected as the primary