WordPress at a glance
action is not described

comment_form_top action-hook . WP 3.0.0

Fires at the top of the comment form, inside the form tag.

Usage

add_action( 'comment_form_top', 'action_function_name_66' );
function action_function_name_66(){
	// action...
}

Where the hook is called

comment_form()
wp-includes/comment-template.php 2328
do_action( 'comment_form_top' );

Where the hook is used (in WP core)

Does not used.