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:
- manually lists of related posts
- post series
- rented houses connected to agency
- etc.
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/