In common^W my sense, controller is a thing that rules the views in a non-data plane. E.g. in master-detail scheme someone has to switch detail datasets once user selects new master row. Controller is the one responsible, cause data has no knowledge of how it is presented — master-detail vs. tree-unfold vs. edit-dialog. Controller emerges from the fact that a single data item can be presented on many different forms with different behavior rules. The last sentence is pretty straightforward on that.