this hook allows the registration of custom post statuses in Subscribe2 so that notifications can be sent for transitions from these statuses to published content. For example, this is useful when using the Edit Flow plugin which creates some custom statuses

function my_post_statuses($statuses) {
    $statuses[] = 'assigned';
    $statuses[] = 'in-progress';
    return $statuses;
}
add_filter('s2_post_statuses', 'my_post_statuses');