The IsActivityFinished property is a property which can be set to true to tell the given Screen that it should no longer be active. Setting IsActivityFinished to true can do a number of things depending on the state of the Screen:
If the Screen is the ScreenManager's CurrentScreen, then this method will result in the ScreenManager moving to the Screen's NextScreen.
If the Screen is a Popup Screen, then the Screen will simply destroy itself and remove itself from its parent's mPopups list.
If the Screen is loading another Screen asynchronously, then setting IsActivityFinished to true will destroy the current Screen and move to the asynchronously-loaded Screen.