Which criterion is most important when using IBM Refactoring Assistant to identify a candidate service boundary?

Think of it like working hands-on with IBM mainframe systems — the answer here is 'Selecting program clusters with high internal cohesion and low external coupling to other clusters'. Service boundaries should follow high cohesion and low coupling: programs that primarily call each other and share data stores form a natural service, minimizing dependencies across the boundary. On real mainframe exams, this concept comes up repeatedly so make sure you have it locked in.