The measurement process TARGET is the “true” value of the part. It is desired that any individual reading be as close to this value as(economically) possible. Unfortunately, the true value can never be known with certainty. However, uncertainty can be minimized by using a reference value based on a well defined operational definition of the characteristic, and using the results of a measurement system that has higher order discrimination and traceable to NIST. Because the reference value is used as a surrogate for the true value, these terms are commonly used interchangeably. This usage is not recommended.
source : Analysis of measurement systems
