"Total unification of the domain model for a large system will not be feasible or cost-effective." #readingToday
The internal consistency of a model, such that each term is unambiguous and no rules contradict, is called unification. A model is meaningless unless it is logically consistent. In an ideal world, we would have a single model spanning the whole domain of the enterprise. This model would be unified, without any contradictory or overlapping definitions of terms. Every logical statement about the domain would be consistent.
But the world of large systems development is not the ideal world. To maintain that level of unification in an entire enterprise system is more trouble than it is worth. It is necessary to allow multiple models to develop in different parts of the system, but we need to make careful choices about which parts of the system will be allowed to diverge and what their relationship to each other will be. We need ways of keeping crucial parts of the model tightly unified. None of this happens by itself or through good intentions. It happens only through conscious design decisions and institution of specific processes. Total unification of the domain model for a large system will not be feasible or cost-effective.
Domain-Driven Design, Eric Evans