woocommerce_checkout_billing │ action-hook │ WC 1.0

Usage

add_action( 'woocommerce_checkout_billing', 'wp_kama_woocommerce_checkout_billing_action' );

/**
 * Function for `woocommerce_checkout_billing` action-hook.
 * 
 * @return void
 */
function wp_kama_woocommerce_checkout_billing_action(){

	// action...
}

Where the hook is called

<?php do_action( 'woocommerce_checkout_billing' ); ?>

Where the hook is used in WooCommerce

add_action( 'woocommerce_checkout_billing', array( self::$instance, 'checkout_form_billing' ) );