It's implicit. Not only software needs iteration. Design documents also need iteration, because there's always something that was overlook, something that just

It's implicit. Not only software needs iteration. Design documents also need iteration, because there's always something that was overlook, something that just escaped common sense, and so on. In other words: Code can have bugs. Design documents also can have bugs, and the latter are almost never fixed. Why is that? Because fixing a design document involves negotiations and changing budgets.