It is a boolean property which influences on indicator calculation. When true, it means that indicator script does not calculate any values, even if new data came, until someone outside will ask it for new calculated data. If developer wants the script recalculate on each input data change he should set it to false. By default this parameter is set to true. If an indicator is without plots and user puts it onto the chart, by default, it will never receive OnCalculate event because there will be no need of calculating data. So, if user wants to receive OnCalculate event in this case he has to set IsLazyCalculation to true.