The set of objects accesible to the user of an application.
A MobileSystem can be any object (or collection of objects) accessible to an application through navigation and/or instantiated from other objects.
The system is also called set of "Domain Objects" (or Model Objects).
It is reccomended to have a class to implement bindings and basic coordination methods/heuristics to make UI specs more readable.
Note that it is not required to implement the system as instance of a specific class! The mobile framework will NOT send messages to the system. The interaction with the system is driven by events and actions on the UI.