woocommerce_before_checkout_billing_form action-hook . WC 1.0

Usage

add_action( 'woocommerce_before_checkout_billing_form', 'action_function_name_4421' );
function action_function_name_4421( $checkout ){
	// action...
}

$checkout

-

Where the hook is called

<?php do_action( 'woocommerce_before_checkout_billing_form', $checkout ); ?>

Where in WP core the hook is used WooCommerce