Requirement (Software Engineering)

Updated 22 Jul 2026

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def
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  a statement of what a system must do or a constraint it must satisfy
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  may range from a high-level abstract statement of a service to a detailed mathematical functional specification
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dual function
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  may be the basis for a bid for a contract — must be open to interpretation by multiple vendors
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  may be the basis for the contract itself — must be defined in detail
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  both documents may be called "requirements"
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requirements abstraction (Davis)
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  requirements must be abstract enough that multiple contractors can bid, yet detailed enough that the client can validate what will be built
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user vs system requirements
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  user requirements
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    written for customers
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    expressed in natural language or with diagrams
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    describes services the system provides and its operational constraints
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    should be understandable by system users without detailed technical knowledge
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    e.g., "The application must be able to store documents securely in large volumes"
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  system requirements
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    written for technical teams
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    structured, detailed descriptions of functions, services, and operational constraints
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    defines what should be implemented; may be part of a contract
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    serves as a basis for designing the system
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    e.g., "The system must support file storage up to 1 GB per user with AES-256 encryption"
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imprecision problems
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  imprecisely stated requirements may be interpreted differently by developers and users
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  leads to misaligned expectations and rework
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completeness and consistency
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  complete — should include descriptions of all required facilities
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  consistent — no conflicts or contradictions among requirement descriptions
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  in practice, truly complete and consistent requirements are rare due to system complexity
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related

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