WordPress at a glance
action is not described

signup_header action-hook . WP 3.0.0

Fires within the head section of the site sign-up screen.

Usage

add_action( 'signup_header', 'action_function_name_3022' );
function action_function_name_3022(){
	// action...
}

Where the hook is called

do_signup_header()
wp-signup.php 28
do_action( 'signup_header' );

Where the hook is used (in WP core)

Does not used.