WordPress at a glance
action is not described

register_form action-hook . WP 2.1.0

Fires following the 'Email' field in the user registration form.

Usage

add_action( 'register_form', 'action_function_name_4520' );
function action_function_name_4520(){
	// action...
}

Where the hook is called

In file: /wp-login.php
register_form
wp-login.php 801
do_action( 'register_form' );

Where the hook is used (in WP core)

Does not used.