I think that’s commonly called “Code Folding” (or “Text folding”)
And it is called Outline by EmEditor.
EmEditor Help – How to – Plug-ins – Use Outline Plug-in
The Outline plug-in is installed by default with EmEditor Professional.
There are two major functions in this plug-in:
1.) Outlining in the custom bar
A tree view appears when you click the on the Plug-ins bar.
Or on the Tools menu, point to Plug-ins, and then click Outline.
In the Outline custom bar, the outline appears as a tree.
…2.) …
[To customize the plugin] right-click on the Outline button on the Plug-ins bar, select Properties, …
Find me: “Code Folding” “CodeFolding” “Code-Folding” “Code-Faltung” “Text folding” “Textfolding” “Text-folding”
From Wikipedia, the free encyclopedia
Code folding is a feature of some text editors, source code editors and IDEs that allows the user to selectively hide
and display sections of a currently-edited file as a part of routine edit operations. This allows the user to manage
large amounts of text while viewing only those subsections of the text that are specifically relevant at any given time.
.