Your "what" is my "how": iteration and hierarchy in system design

Systems are naturally constructed in hierarchies, in which design choices made at higher levels of abstraction levy requirements on system components at the lower levels. Thus, whether an aspect of a system is a design choice or a requirement largely depends on your vantage point within the system components' hierarchy. Systems are also often constructed from the middle-out rather than top-down; compatibility with existing systems and architectures and availability of specific components influence high-level requirements. Requirements and architectural design should be more closely aligned: requirements models must account for hierarchical system construction and architectural design notations must better support requirements specification for system components.