Field instruments are calibrated using master instruments. Master instruments are instruments with more accuracy and greater repeatability. Master instruments are calibrated periodically at external laboratories. And the instruments in these labs too are calibrated at another place. Such a chain is called calibration chain. Identification is provided on all these of the instrument used to calibrate it and that is called traceability. In India, mostly instruments are calibrated and are traceable to NPL (National Physics Laboratory)
Figure below represents the block diagram of calibration chain and traceability.