Pigrecolab

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.

Primary Category

WordPress plugin for setting and displaying primary categories for posts and custom post types. This is a fork from Jacob McKinney's Primary Cate …

theode

I once thought I need different primary menus for different viewers. 
In best case those users have a GET paramter which decides which menu has to be shown.
For Example: 
You go into WordPress and add a menu with a name of your choice. I choosed “music”. 
Now if you want somebody view the music menu you have the parameter http://www.example.com/?view=music. 
You can do this with every menu you have.
In case you leave it empty nothing happen at all.

GET Different Menus

Have different primary menus for different viewers.

Primary Category vs GET Different Menus