Page History

...

• The lifetime of a windowId is smaller than a session but greater than an individual request processing lifecycle
• A window-id always is unique in the session of the user. A session contains multiple windowIds, but only exactly 1 window-id >per< browser tab.
• A client window is always associated with exactly one UIViewRoot instance at a time, but it may display many different UIViewRoots during its lifetime.
• The windowId must not be shared across browser tabs. 
• The windowId must not be included in the ViewState, because it needs to be available before the ViewState has been decoded.
• The windowId must be the anchor to which the Flash is hitched.

...