Accuracy classes are defined and used in IEC

IEC

-Organisations:* Independent Electoral Commission * Independent Electrical Contractors, a U.S. national trade association.* Institut d'Estudis Catalans , a Catalan academic institution....

 and ANSI

Ansi

Ansi is a village in Kaarma Parish, Saare County, on the island of Saaremaa, Estonia....

 standards. Classes are denoted by either a letter or percentage. For example, Class B is a temperature accuracy

Accuracy and precision

In the fields of science, engineering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which...

 from IEC-751 that requires accuracy of +/- 0.15 degrees Celsius

Celsius

Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...

. Class 0.5 is an ANSI C12.20 accuracy class for electric meters with accuracy of +/- 0.5%.

Typically, accuracy is measured against a nominal (maximum) rated value and may vary at lower values.

Accuracy classes such as IEC's 0.15s are a 'special' high accuracy class.