Classes of accuracy refers to the accuracy of the calibration weights themselves. These classes are determined alongside weighing tolerances i.e. the amount of permissible errors either above or under the target weight. In this instance, tolerances are defined by the International Organisation of Legal Metrology’s (OIML) R 111-1 recommendations.
Different types of weighing instruments might require different classes, depending on how precise the measurements need to be. A laboratory balance, for example, would need to measure finer items to a high precision, so would likely require a weight with an accuracy class of E2 and higher. A bench scale in a bakery, though, might not need that level of precision and so an M1 class weight might suffice for testing.
See below for a comparison of how accuracy classes differ:
MPE = Maximum permissible error
For comprehensive guidance on accuracy classes and tolerances, you can refer to the OIML R 111-1 recommendations.