allout-distinctive-bullets-string is a variable defined in `allout.el'.
Its value is

  • Automatically becomes buffer-local when set.
  • This variable is safe as a file local variable if its value
    satisfies the predicate `stringp'.

Persistent outline header bullets used to distinguish special topics.

These bullets are distinguish topics with particular character.
They are not used by default in the topic creation routines, but
are offered as options when you modify topic creation with a
universal argument (C-u), or during rebulleting (M-x allout-rebullet-current-heading).

Distinctive bullets are not cycled when topics are shifted or
otherwise automatically rebulleted, so their marking is
persistent until deliberately changed. Their significance is
purely by convention, however. Some conventions suggest

`(' - open paren -- an aside or incidental point
`?' - question mark -- uncertain or outright question
`!' - exclamation point/bang -- emphatic
`[' - open square bracket -- meta-note, about item instead of item's subject
`"' - double quote -- a quotation or other citation
`=' - equal sign -- an assignment, some kind of definition
`^' - carat -- relates to something above

Some are more elusive, but their rationale may be recognizable:

`+' - plus -- pending consideration, completion
`_' - underscore -- done, completed
`&' - ampersand -- addendum, furthermore

(Some other non-plain bullets have special meaning to the
software. By default:

`~' marks encryptable topics -- see `allout-topic-encryption-bullet'
`#' marks auto-numbered bullets -- see `allout-numbered-bullet'.)

See `allout-plain-bullets-string' for the standard, alternating

You must run `set-allout-regexp' in order for outline mode to
adopt changes of this value.

DO NOT include the close-square-bracket, `]', on either of the bullet

You can customize this variable.