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Flex Fields is a WordPress plugin that allows users to easily add custom meta boxes for post types, page templates, taxonomies, post statuses, and post taxonomies. These custom meta boxes can be utilized in code using the flex_field() function with the key parameter.

Flex Fields

Flex Fields is a WordPress plugin to add custom meta boxes for post types, pages, taxonomies, and more, using the flex_field() function.

Amaury Balmer

Nicolas Juen

This plugin allow to build relation between 2 custom types (posts, page, custom), very useful for manage related content on CMS type website.
A few example use cases:
<ul>
<li>manually lists of related posts</li>
<li>post series</li>
<li>rented houses connected to agency</li>
<li>etc.</li>
</ul>
Relationships are created from a settings page from the administration console.
This plugin can not do relationships with users, you should test this excellent plugin made by scribu 
https://wordpress.org/extend/plugins/posts-to-posts/

Relations Post Types

This plugin allow to build relation between 2 custom types (posts, page, custom), very useful for manage related content on CMS type website.

Flex Fields vs Relations Post Types