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A current transformer's class, accuracy class, to be more precise, signifies the error of the CT. 

For metering CTs, the designated accuracy classes are 0.1, 0.2, 0.5, 1.0, 3.0 & 5.0; 

For tariff metering, the classes are 0.2S & 0.5S

For general protection, the classes are 5P, 10P & 15P with ALF (Accuracy Limiting Factor) of 5, 10, 15, 20 & 30.

For Unit Protection Schemes, the classe is Class PS.- See more at: http://myelectrical.com/questions/view/question/id/86#sthash.I3uzL1if.dpuf

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Metering: 0.2S for Revenue Metering, 0.2 in  Measuring in Power  Plants & industries for internal

Purpose, 1 for Domestic Reading

Differential & REF Protection: PS

Protection: Like over current, earth fault, Reverse Power, under Power etc., 3P5, 3P10, 5P5, 5P10,

5P20.

The above classes are used practically.

- See more at: http://myelectrical.com/questions/view/question/id/86#sthash.I3uzL1if.dpuf

Accuracy class:0.2S more accurate,for protection see data sheet. CT s, accuracy class is typically, 0.2, 0.5, 1 or 3. This means that the errors have to be within the limits specified in the standards for that particular accuracy class. The metering CT has to be accurate from 5% to 120% of the rated primary current, at 25% and 100% of the rated burden at the specified power factor. In the case of protection CT s, the CT s should pass both the ratio and phase errors at the specified accuracy class, usually 5P or 10P, as well as composite error at the accuracy limit factor of the CT