This is a calculator which uses Reverse Polish Notation, or RPN. A full write-up in Japanese (Google Translate link) carries an impressive level of detail about the project, but in short ...

Risk Priority Number (RPN) is an integral part of the creation of a Failure Mode and Effect Analysis (FMEA) document. However, it is also used widely wherever a risk management strategy is deployed.

[Suikan] has taken one of these plentiful models and done something special for it, creating a new mainboard, and a firmware which transforms it into a reverse Polish, or RPN, scientific calculator.