Accuracy classes are defined and used in IEC and ANSI standards. Classes are denoted by either a letter or percentage. For example, Class B is a temperature accuracy from IEC-751 that requires accuracy of +/- 0.15 degrees Celsius. 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.