RPNCalcSeries Engine is the runtime environment, where all RPNCalcSeries calculators run. It's freeware and you need to download it just once. From time to time it's a good idea to check for updates in the engine. RPNCalcSeries calculators are a series of specialized calculators sold separately. You can find them as a Single or Multi-Module calculators. Multi-Module calculators conveniently group single calculators in modules like Mathematics, Finance, etc.., with a very low price, considering the sum of the single calculators prices that are part of the multi-module calculator.