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R14 / Generalization

Subsystem Element is a Relationship or Class

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Relationships and Classes are traditionally documented and managed as subsystems.

Take the Domain Subsystem right here for example. Both Classes and Relationships are displayed on a separate sheet/layout and are documented with class and relationship descriptions as a complete subsystem. And the Classes and Relationships are identified, in part, in a numbering range (1-24) as of this writing.

By contrast, Spanning Elements tend to be documented, numbered, and managed across an entire Domain without regard to Subsystem.