The reference value is the known and correct measurement for the standard part. It serves as a master value for comparison during measurement system analysis. For example, you have a reference part with a known weight of 0.025g that you use to calibrate your scales.
Ideally, the reference value should be close to the center of the tolerance zone for the characteristic that you measure.
Reference values can be determined in many ways, depending on industry standards and company and customer expectations. Usually a reference value is determined by taking an average of repeated measurements from more accurate measuring equipment, or by using a lab-certified standard.