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Short answer: The calibration lab has fewer false rejects, and everything is PASS/FAIL, no unknowns.
Long answer: ISO-17025:2005 is a European plan which was once report the calibration, and let the customer decide if the DUT is OK. Z540.3 is a process where the calibration facility can agree with the customer, to an acceptable false pass tolerance. The <2% has been put forth as a model to follow. Full guard banding causes a false reject, which could be viewed to high for some customers. Z540.3 allows for a course for guard banding. Actually there are several defined courses in use. It clearly breaks away from the notion of TL=SL-Uc, where Test Limit, Spec Limit and combined Uncertainties are known. Z430.3 puts forth the idea of all tests are PASS/FAIL, and no more PASS,PASS*,FAIL*,FAIL.
Long answer: ISO-17025:2005 is a European plan which was once report the calibration, and let the customer decide if the DUT is OK. Z540.3 is a process where the calibration facility can agree with the customer, to an acceptable false pass tolerance. The <2% has been put forth as a model to follow. Full guard banding causes a false reject, which could be viewed to high for some customers. Z540.3 allows for a course for guard banding. Actually there are several defined courses in use. It clearly breaks away from the notion of TL=SL-Uc, where Test Limit, Spec Limit and combined Uncertainties are known. Z430.3 puts forth the idea of all tests are PASS/FAIL, and no more PASS,PASS*,FAIL*,FAIL.

