comment style that tells text editors to fold sections of text

I noticed that the developer of kitty terminal uses this style of comments I have rarely if ever seen elsewhere:

 sh

    
outside
#: section {{{
inside
#: }}}


  

Kate text editor recognize sections for purposes of highlighting, folding etc. It's really nice for me because I sometimes I have a difficult time navigating large text files. And it lets you nest them.

• what is this called?
• are there other ways to do it?
• is it standard among text editors? I believe Kovid the dev for kitty is a vim guy so presumably there is support there also.
• why don't more people use it? are there problems?

Screenshot that shows the code folding.

• Cursor is at the end of line 7 so the whole section line 7-22 is highlighted
• lines 12-16 are folded in a 3rd level comment
• I also included tab indents just to make it easier to see what's going on (Kate treats it the same way regardless of indents)
• Highlighting/Mode > Scripts > Bash

I also like his style of distinguishing between narrative comments (starting with #:) and commented-out code (starting with #). Although in my example, Kate doesn't treat them differently. Is there a term for this? Any conventions, support etc?

For a real world example, see sample kitty.conf file provided on project website.