WordPress at a glance
action is not described

woocommerce_checkout_billing action-hook . WC 1.0

Usage

add_action( 'woocommerce_checkout_billing', 'action_function_name_7533' );
function action_function_name_7533(){
	// action...
}

Where the hook is called

In file: /templates/checkout/form-checkout.php
woocommerce_checkout_billing
woocommerce/templates/checkout/form-checkout.php 40
<?php do_action( 'woocommerce_checkout_billing' ); ?>

Where in WP core the hook is used WooCommerce

woocommerce/includes/class-wc-checkout.php 58
add_action( 'woocommerce_checkout_billing', array( self::$instance, 'checkout_form_billing' ) );