"expired session" vs "expired document halder"

> Hi guys,
>
> I am developing my own seaside session tracking subclass
> WAHandlerTrackingStrategy (which uses cookies but in a different way)  but I
> am finding problems to understand a couple of comments and code I found
> around. For example:
>
> "Don't delete the session cookie if we have an expired document halder.
> Only delete the session cookie if we actually have an expired session."
>
> And:
>
> As ususal care has to be taken that this is done only for expired sessions
> and not for expired document handlers
>
> And:
>
>  Some tracking methods may not be appropriate for some kinds of handlers
> (eg. a WADocumentHandler should never be tracked using a cookie). Some
> requests may have multiple keys (eg. a query field for a WADocumentHandler
> and a cookie for a WASession).
>
>
> So.. I am not sure what does mean. I am interested in tracking SESSIONS, not
> document handlers. And I must clean stuff at session expiration only. I
> don't know what the document handler expiration is about nor how to
> distinguish it from session expiration. So then I cannot know when to clean
> and when to not clean.
>
>
> Any clarification?