EdwardBock

Palasthotel

You can add relations between posts with a new meta box in post editor.
<h3>Arbitrary section</h3>

Content Relations

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.

Content Relations vs Relations Post Types