allout-mode-leaders is a variable defined in `allout.el'.
Its value is

Specific allout-prefix leading strings per major modes.

Use this if the mode's comment-start string isn't what you
prefer, or if the mode lacks a comment-start string. See
`allout-use-mode-specific-leader' for more details.

If you're constructing a string that will comment-out outline
structuring so it can be included in program code, append an extra
character, like an "_" underscore, to distinguish the lead string
from regular comments that start at the beginning-of-line.