What are the rules applying to the classification of measuring instruments?

When measuring equipment is described in a standard or in regulatory texts (e.g. for legal metrology), the permissible errors are defined according to the type of instrument and its scope of measurement. The problem is quite different for users of equipment that is not described in any standards or regulations. In this case, users define the maximum errors tolerated by the product or the process according to their own particular requirements.

Note that the tolerance on the product and the maximum permissible error of the measurement are not the same thing.