This allows you to specify the ID generated by Helpful for the user to identify. This ID is stored in a PHP session if cookies are not allowed. From the ID no user data can be read out, it is therefore GDPR conform.

/**
 * Here you can generate your own ID for your users. By default,
 * a unique random string is created. This ID is automatically stored
 * in a cookie or a session, depending on what is currently working.
 *
 * @param string $user_id user id string.
 * @return string
 */
function custom_function( $user_id ) {
	$user_id = 'unique_user_id';
	return $user_id;
}

add_filter( 'helpful_user_string', 'custom_function' );