The Problem-Domain Component (PDC) is the part of the program that relates just to the problem itself, not to dealing with data (Data Management Component (DMC)) nor with the human interface (Human Interface Component (HIC)). It's how Coad & Nicola break up the OOA, OOD, and OOP cycles. It makes sense from general object-oriented methodology. The idea of O-O is to focus on modeling the real-world domain in terms of objects. The PDC is just on that part -- the stuff that corresponds to the real world.