Paolo Bertinetti

QRMemo is a versatile WordPress plugin that enables you to automatically generate and display QR codes at the end of specific post types. The QR codes added automatically contain the URL of the page or post they are appended to, making it easy for users to share or save the page.
The plugin also supports generating QR codes using a shortcode, allowing you to insert a QR code with custom text anywhere in your content.
Key Features: 
* Automatically add QR codes containing the page URL to the end of posts, pages, and other custom post types. 
* Customize which post types should display QR codes. 
* Generate QR codes via shortcode, specifying custom text, size, margin, and alignment.
Libraries Used: 
* <a href="https://github.com/endroid/qr-code" rel="nofollow ugc">Endroid QR Code</a> – A powerful library for generating QR codes in PHP.
<h3>License</h3>
This plugin is licensed under the GPLv2 or later. For more details, see <a href="https://www.gnu.org/licenses/gpl-2.0.html" rel="nofollow ugc">https://www.gnu.org/licenses/gpl-2.0.html</a>.

QRMemo

QRMemo is a WordPress plugin that adds QR codes to the end of selected post types and allows generating QR codes via shortcode.

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.

QRMemo vs Relations Post Types