WordPress at a glance
action is not described

woocommerce_(id)_shipping_add_rate action-hook . WC 1.0

Developers can add additional flat rates based on this one via this action since @version 2.4.

Previously there were (overly complex) options to add additional rates however this was not user. friendly and goes against what Flat Rate Shipping was originally intended for.

Usage

add_action( 'woocommerce_(id)_shipping_add_rate', 'action_function_name_2384', 10, 2 );
function action_function_name_2384( $that, $rate ){
	// action...
}
$that
-
$rate
-

Where the hook is called

WC_Shipping_Flat_Rate::calculate_shipping()
woocommerce_(id)_shipping_add_rate
woocommerce/includes/shipping/flat-rate/class-wc-shipping-flat-rate.php 216
do_action( 'woocommerce_' . $this->id . '_shipping_add_rate', $this, $rate );

Where in WP core the hook is used WooCommerce

woocommerce/includes/shipping/legacy-flat-rate/class-wc-shipping-legacy-flat-rate.php 41
add_action( 'woocommerce_flat_rate_shipping_add_rate', array( $this, 'calculate_extra_shipping' ), 10, 2 );