From PoSa:
''The PresentationAbstractionControl architectural pattern (PAC) defines a structure for interactive software systems in the form of a hierarchy of cooperating agents.
Every agent is responsible for a specific aspect of the application's functionality and consists of three components: presentation, abstraction, and control.
This subdivision separates the human-computer interaction of the agent from its functional core and its communication with other agents.''
See http://www.idemployee.id.tue.nl/p.markopoulos/downloadablePapers/PhDThesisPanosMarkopoulos.pdf page 27 for further explanation of the PAC pattern, as well as http://en.wikipedia.org/wiki/Presentation-abstraction-control
See also HierarchicalModelViewController