allout-header-prefix 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'.

Leading string which helps distinguish topic headers.

Outline topic header lines are identified by a leading topic
header prefix, which mostly have the value of this var at their front.
Level 1 topics are exceptions. They consist of only a single
character, which is typically set to the `allout-primary-bullet'.

You can customize this variable.