IwGx and its subcomponents use three different types of surfaces.
Device, which holds the width and height of the device, without worrying about the screen orentation.
Screen, which is the same as Device but switches the width and height when the device orientation is changed.
Surface, which is made by the programmer. It's up to you to create surfaces. A screen can have several surfaces or none at all, like images. Its a rectangular object used for the UI.
For more information you can look it up in the documentation:
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