allout-exposure-change-hook is a variable defined in `allout.el'.
Its value is

Abnormal hook run after allout outline subtree exposure changes.
It is run at the conclusion of `allout-flag-region'.

Functions on the hook must take three arguments:

- FROM -- integer indicating the point at the start of the change.
- TO -- integer indicating the point of the end of the change.
- FLAG -- change mode: nil for exposure, otherwise concealment.

This hook might be invoked multiple times by a single command.

You can customize this variable.