Guardband is the offset from a specification that will determine an acceptable limit. The guardband is used to make a “Pass” or “Fail” decision on the specification and it is also used to perform adjustment if needed. Guardbanding takes in account all of the uncertainties in a measurement system.
The ISO/IEC 17025 standard requires that guardbanding be applied for each test point when the test test uncertainly ratio (TUR) is less than 4:1. If guardbanding leads to a situation where it is imposible to determine, within the confidence interval, the statement of compliance or non-compliance for the measurand. In this situation, the declaration of conformoty is "undetermined". The reason Guardband is used is to reduce the risk even further of accepting out-of-tollerance units or rejecting a unit that is in-tollerance.