Document chaining is navigating from one qlikview application to other. While doing that we can optionally pass the selection from the first application to the other. You can do it by setting an action either in text object or button to open a qlikview application.
You have to mention the Document path,
Transfer State - Will pass the selection in the current application to the other (Column name must be same in both the application)
Apply State on top of current - Will apply the selection on top of the current selection in the other
You need to keep in mind that it won't work in IE client. It will work only in browsers (AJAX Client)
One of the scenario where you can use is creating a front page or a dashboard page for all the users and route the user to specific application with specific selection. This way you can maintain only single URL for all the users. And the application open time will reduce when opening with some selection.
Hope it helps somewhat...