allout-file-xref-bullet is a variable defined in `allout.el'.
Its value is
"@"
This variable is safe as a file local variable if its value
satisfies the predicate `string-or-null-p'.
Documentation:
Bullet signifying file cross-references, for `allout-resolve-xref'.
Set this var to the bullet you want to use for file cross-references.
You can customize this variable.