NOTE 1 In the Error Approach to describing measurement, a true quantity value is considered unique and, in practice, unknowable. The Uncertainty Approach is to recognize that, owing to the inherently incomplete amount of detail in the definition of a quantity, there is not a single true quantity value but rather a set of true quantity values consistent with the definition. However, this set of values is, in principle and in practice, unknowable. Other approaches dispense altogether with the concept of true quantity value and rely on the concept of metrological compatibility of measurement results for assessing their validity.
NOTE 2 In the special case of a fundamental constant, the quantity is considered to have a single true quantity value.
NOTE 3 When the definitional uncertainty associated with the measurand is considered to be negligible compared to the other components of the measurement uncertainty, the measurand may be considered to have an “essentially unique” true quantity value. This is the approach taken by the GUM and associated documents, where the word “true” is considered to be redundant.
ANNOTATION (informative) [5 June 2014] Here "quantity value" can be replaced with "value" without ambiguity: "value consistent with the definition of a quantity".
ANNOTATION (for Note 1) (informative) [5 June 2014]: Two reasons that the set of true values is unknowable are: 1) the measurement model used in a measurement may be incorrect, and 2) a mistake may be made in performing the measurement.