XPE allows values to be taken from on screen controls such as textboxes, sliders and radio buttons and then passed to the application code using a simple function call. The parameters to the function being defined by the application programmer. On return from the function, the results can easily be applied to the screen controls. The most important point to observe here is that the application programmer is in complete control of defining the interface between the application software and the GUI, the programmer is not confined to using a bizare protocol to access the values entered by the user.
Further more this mechanism allows intelligence to be added to GUI objects allowing them to qualify user entry and invoke dialogs where necessary. This greatly reduces the burden on the application code itself since user interaction becomes trivial.