I think the hard part is where the customer doesn't really know everything about their business process: it's extremely common to have a partial view of a compl

I think the hard part is where the customer doesn't really know everything about their business process: it's extremely common to have a partial view of a complex system. The process of requirements discovery involves asking questions, pointing out inconsistencies, and setting priorities. Only at that point do you sort of have an agreement about the desired work and even then there are gaps that need iteration.