Outline minor mode is a buffer-local minor mode which provides the same commands as the major mode, Outline mode, but can be used in conjunction with other major modes. You can type M-x outline-minor-mode to toggle Outline minor mode in the current buffer, or use a file-local variable setting to enable it in a specific file (see Local Variables in Files).
The major mode, Outline mode, provides special key bindings on the
C-c prefix. Outline minor mode provides similar bindings with
C-c @ as the prefix; this is to reduce the conflicts with the
major mode’s special commands. (The variable
outline-minor-mode-prefix controls the prefix used.)
If outline-minor-mode-use-buttons is non-nil, Outline
minor mode will use buttons at the beginning of the heading lines, in
addition to ellipsis, to show that a section is hidden. Clicking the
mouse on the button toggles display of the section. If the value of
this variable is insert, the buttons are inserted directly into
the buffer text, so RET on the button will also toggle display
of the section, like a mouse click does. Using the value insert
is not recommended in editable buffers because it modifies them.
If the value is in-margins, Outline minor mode will use the
window margins to indicate that a section is hidden. The buttons are
customizable as icons (see Icons).
If the outline-minor-mode-cycle user option is
non-nil, the TAB and S-TAB keys that cycle
the visibility are enabled on the outline heading lines
(see outline-cycle). TAB cycles hiding,
showing the sub-heading, and showing all for the current section.
S-TAB does the same for the entire buffer.