Anything related to dynamic placeholders, which includes the practice of having multiple renderings on a page that includes the same placeholder key, and trying to assign a specific rendering to one specific placeholder key from those renderings.

Dynamic placeholders helps us adding the same placeholder name several times. We can do this across multiple renderings, and we can even use the same placeholder multiple times in a single rendering.

Dynamic placeholders mean that:

  1. Unique keys are guaranteed across different renderings and within one rendering.
  2. We can build functionality that generates multiple placeholders within the same rendering dynamically.

https://doc.sitecore.net/sitecore_experience_platform/developing/developing_with_sitecore/dynamic_placeholders

https://jammykam.wordpress.com/2017/10/17/sitecore-9-dynamic-placeholders/

https://www.sitecorenutsbolts.net/2017/10/17/Sitecore-9-Dynamic-Placeholders-At-Last/