outline-heading-alist

outline-heading-alist is a variable defined in `outline.el'.
Its value is

nil

• Automatically becomes buffer-local when set.

Documentation:
Alist associating a heading for every possible level.
Each entry is of the form (HEADING . LEVEL).
This alist is used two ways: to find the heading corresponding to
a given level and to find the level of a given heading.
If a mode or document needs several sets of outline headings (for example
numbered and unnumbered sections), list them set by set and sorted by level
within each set. For example in texinfo mode:

(setq outline-heading-alist
'(("@chapter" . 2) ("@section" . 3) ("@subsection" . 4)
("@subsubsection" . 5)
("@unnumbered" . 2) ("@unnumberedsec" . 3)
("@unnumberedsubsec" . 4) ("@unnumberedsubsubsec" . 5)
("@appendix" . 2) ("@appendixsec" . 3)...
("@appendixsubsec" . 4) ("@appendixsubsubsec" . 5) ..))

Instead of sorting the entries in each set, you can also separate the
sets with nil.