We have session variables, site variables, local variable etc however i feel i feel we should also have flow variables defined where in if we define a variable at literal flow level then its value is avaible to all the page within the flow. Each page can view/edit/delete the value.
In terms for applications we genraly follow the modular approach where we want to devide the application into flows so this variable will be handy.
Just a thought. :)