Specification Limits are the engineering-defined boundaries that define acceptable product performance — derived from design requirements, customer specifications, and process capability studies, specification limits define the range within which a measured parameter must fall for the product to be acceptable.

Specification Limit Types

• USL (Upper Specification Limit): The maximum acceptable value — exceeding USL means the product exceeds tolerance.
• LSL (Lower Specification Limit): The minimum acceptable value — below LSL means the product is under tolerance.
• Bilateral: Both USL and LSL exist — the parameter must fall within the range [LSL, USL].
• Unilateral: Only one limit — e.g., defect density only has a USL (lower is always better).

Why It Matters

• Different from Control Limits: Spec limits come from the CUSTOMER (what's needed); control limits come from the PROCESS (what's achieved).
• Capability: The relationship between spec limits and process variation defines capability (Cp, Cpk).
• Disposition: Product outside spec limits is rejected, reworked, or used-as-is with customer concession.

Specification Limits are the customer's requirements — the engineering boundaries that define acceptable product performance, distinct from process control limits.