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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.