Accuracy.

If you want to verify the actual dimension of a part, you would use a calibrated Vernier Caliper. Should you want to verify the the actual diameter of a part, you would use a Micrometer.

The same is true for a Thread gage. All of these hand measuring instruments can be calibrated by your inspection department.

see this link:

http://www.wihatools.com/411serie_calipers.htm

Mav