The GuiLayer mediates between the RepresentationLayer and the GuiToolkit.

The GuiLayer is what should be ultra-thin, but tested anyway.

The RepresentationLayer converts LogicLayer stuff into the representation (format, data model shape, locale, etc.) that the GuiLayer uses.

Together, the GuiLayer and RepresentationLayer form the PresentationLayer.

The LogicLayer is everything else - the BusinessLayer, PersistenceLayer, etc.