The actual realization of a unit for any physical quantity requires the following:
Maintenance under specified conditions of a prototype of it, called its primary standard.
The unit may be equal to, a multiple or submultiples of the standard for practical reasons.The maintenance of exact standards is necessary for industrial purposes. Industries cover a wild field of scientific application, and many of them have attained a high degree of perfection. To exist in the competitive market, they must constantly improve their products. For this purpose, they might need high precision tools.
The standards are being maintained by international bodies only weight and measures. In India, National Physical Laboratory at New Delhi is involved in the maintenance of standards. In addition to NPL at New Delhi, there are many religion and state level laboratories that help in the maintenance of the national standards. The NPL, New Delhi calibrates the measurement standards at all these laboratories. The weights and other measures used in daily life and in industry are tested in these laboratories and certified.