It removes assumptions about "good engineering practice." A feature passes if it meets the specified tolerance as measured according to the GPS measurement standards . No implied extra strictness.

ISO 8015 specifies a standard reference temperature for dimensional measurements.

| Aspect | Without ISO 8015 (Implied Envelope) | With ISO 8015 (Explicit Independency) | | :--- | :--- | :--- | | | Controls size + straightness + roundness automatically. | Controls only size (local two-point measurements). | | Required form control | None needed for many sliding fits. | Must add flatness, straightness, circularity tolerances. | | Inspection | Use a ring gauge (envelope check). | Use calipers/micrometer (2-point size check only). | | Risk | Over-constraint, rejecting parts that function. | Under-constraint, accepting parts that fail assembly. |

If you specify a shaft diameter of 20 ± 0.1 mm under ISO 8015, the shaft could theoretically be bent like a banana or have an oval cross-section, as long as every two-point measurement across the feature falls between 19.9 mm and 20.1 mm. To control form (straightness, roundness), you must add separate geometric tolerances.

Unlike the ASME Y14.5 standard (which defaults to the Envelope Principle/Rule #1), ISO 8015 does assume that a part must fit within a perfect envelope of maximum material size. If the designer wants the size tolerance to control the form, they must explicitly indicate this using the Envelope Requirement (symbol Ⓔ) .

Every drawing must assume a specific, unambiguous measurement method (e.g., two-point measurement for size, least-squares for form). No vague "good fit" assumptions.

Understanding ISO 8015: The Foundation of Modern Engineering Tolerances