D Phobos Library Documentation: What is the Internal API for?

[BoQsc]

This is pretty basic question.
If you open [D Library 
Reference](https://dlang.org/phobos/index.html) you are bound to 
see the **Internal API** section in the table of content.

What is the **Internal API** (internal.core, dmd, rt) for and 
when, how and where to use it?

![](https://i.imgur.com/WyemZsG.png)