In DFSS, functional requirements define what the product or service must do and how well it must perform in order to satisfy customer needs and design intent. Their purpose is to establish performance levels that can later be translated into measurable engineering, service, or process characteristics. This is a critical step because customer needs are often expressed in broad or qualitative terms, while functional requirements convert those needs into specific expectations for capability, reliability, speed, capacity, accuracy, or other performance dimensions. Once these requirements are established, the design team can align concepts, specifications, tolerances, and verification methods around them. While functional requirements may also improve communication and reduce redesign indirectly, those are secondary effects rather than their primary purpose. In Black Belt DFSS work, the team must clearly define what the design must achieve before selecting features or building the process that will deliver it. Therefore, the correct answer is A, establish performance levels, because functional requirements provide the performance foundation that guides robust product or service design.

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