Emission credits mean the amount of emission reductions or exceedances, by a heavy-duty engine family, below or above the emission standard, respectively. Emission credits below the standard are considered as positive credits, while emission credits above the standard are considered as negative credits. In addition, projected credits refer to emission credits based on the projected U.S. production volume of the engine family. Reserved credits are emission credits generated within a model year waiting to be reported to EPA at the end of the model year. Actual credits refer to emission credits based on actual U.S. production volumes as contained in the end-of-year reports submitted to EPA. Some or all of these credits may be revoked if EPA review of the end of year reports or any subsequent audit actions uncover problems or errors.