Ok I think I understand this now :
The following placeholder values must appear in the body:
SiteminderReason=$$Reason$$
SiteminderRedirectURL=$$URL$$
Example: Suppose that a web application client request triggers an idle timeout. CA Single Sign-On replaces the placeholder values with IdleTimeoutURL and the URL specified in the value of the IdleTimeoutURL parameter.
But, my question still stands. How does the WebAppClientResponse ACO parameter help with the AJAX request that got interrupted by the siteminder session timeout ??? Sure you can customize the response, but the redirection URL will still make you lose your current page. I don't see how you can smoothly return the AJAX response to the page you were on before you are redirected to the login page.