;;; is for headings only

[tarsius]

In Emacs Lisp by convention three or more semicolons are treated as headings.

From (elisp)Comment Tips

`;;;'
     Comments that start with three semicolons, `;;;', should start at
     the left margin.  We use them for comments which should be
     considered a "heading" by Outline minor mode.  By default,
     comments starting with at least three semicolons (followed by a
     single space and a non-whitespace character) are considered
     headings, comments starting with two or fewer are not.
     Historically, triple-semicolon comments have also been used for
     commenting out lines within a function, but this use is
     discouraged.

     When commenting out entire functions, use two semicolons.

outline-minor-mode and others use this like this.

;;; heading
;;;; subheading
;;;;; subsubheading
;;;;;; and so on

;; comment
;; continued
(code)

[Malabarba]

Might this be the same as #2?

[tarsius]

Ups sorry.