WordPress at a glance
action is not described

woocommerce_order_status_changed action-hook . WC 1.0

Usage

add_action( 'woocommerce_order_status_changed', 'action_function_name_3988', 10, 4 );
function action_function_name_3988( $id, $status_transition_from, $status_transition_to, $that ){
	// action...
}
$id
-
$status_transition_from
-
$status_transition_to
-
$that
-

Where the hook is called

WC_Order::status_transition()
woocommerce_order_status_changed
woocommerce/includes/class-wc-order.php 373
do_action( 'woocommerce_order_status_changed', $this->get_id(), $status_transition['from'], $status_transition['to'], $this );

Where in WP core the hook is used WooCommerce

woocommerce/includes/tracks/events/class-wc-orders-tracking.php 18
add_action( 'woocommerce_order_status_changed', array( $this, 'track_order_status_change' ), 10, 3 );