The following table provides a summary of the requirements for metric weights and mass standards commonly used in chemical analysis1-3. The actual specifications are under the jurisdiction of ASTM Committee E-41 on General Laboratory Apparatus and are the direct responsibility of subcommittee E-41.06 that deals with weighing devices. These standards do not generally refer to instruments used in commerce. Weights are classified according to type (either Type I or Type II) grade (S, O, P or Q) and class (1, 2, 3, 4, 5 or 6). Information on these mass standards is presented to allow the user to make appropriate choices when using analytical weights for the calibration of electronic analytical balances, for making large-scale mass measurements (such as those involving gas cylinders) and in the use of dead weight pressure balances.
CRC Handbook of Chemistry and Physics - 93rd edition