As for how those interfaces are managed? Well, there are a variety of ways. On any given day, I might be carrying out a trade study to determine how the subsystems should interact with each other. Or I might be developing or implementing a requirement that governs how the subsystems should perform. I might be writing a code or spreadsheet to make sure that a system is performing as it should, or I may be writing a test procedure that validates its performance. All of these tasks involve working with different engineers, asking questions, and gathering their assessments. Every day is a busy one, and I never know what will come across my desk when I walk in the door.