allout-default-layout is a variable defined in `allout.el'.
Its value is
(-2 : 0)

Default allout outline layout specification.

This setting specifies the outline exposure to use when
`allout-layout' has the local value `t'. This docstring describes the
layout specifications.

A list value specifies a default layout for the current buffer,
to be applied upon activation of `allout-mode'. Any non-nil
value will automatically trigger `allout-mode', provided
`allout-auto-activation' has been customized to enable it.

The types of elements in the layout specification are:

INTEGER -- dictate the relative depth to open the corresponding topic(s),
-- negative numbers force the topic to be closed before opening
to the absolute value of the number, so all siblings are open
only to that level.
-- positive numbers open to the relative depth indicated by the
number, but do not force already opened subtopics to be closed.
-- 0 means to close topic -- hide all subitems.
: -- repeat spec -- apply the preceding element to all siblings at
current level, *up to* those siblings that would be covered by specs
following the `:' on the list. Ie, apply to all topics at level but
trailing ones accounted for by trailing specs. (Only the first of
multiple colons at the same level is honored -- later ones are ignored.)
* -- completely exposes the topic, including bodies
+ -- exposes all subtopics, but not the bodies
- -- exposes the body of the corresponding topic, but not subtopics
LIST -- a nested layout spec, to be applied intricately to its
corresponding item(s)

(-2 : 0)
Collapse the top-level topics to show their children and
grandchildren, but completely collapse the final top-level topic.
(-1 () : 1 0)
Close the first topic so only the immediate subtopics are shown,
leave the subsequent topics exposed as they are until the second
second to last topic, which is exposed at least one level, and
completely close the last topic.
(-2 : -1 *)
Expose children and grandchildren of all topics at current
level except the last two; expose children of the second to
last and completely expose the last one, including its subtopics.

See `allout-expose-topic' for more about the exposure process.

Also, allout's mode-specific provisions will make topic prefixes default
to the comment-start string, if any, of the language of the file. This
is modulo the setting of `allout-use-mode-specific-leader', which see.

You can customize this variable.