They are an intermediate model between a controller and an automated process or data logger.
They are used where there is a need for on-site physical recording or storage of data – logging (this is a paper record with the date and time, which is very difficult to forge). Often critical process points in the energy industry (recording of important parameters, pressures, temperatures), waste water treatment plants (recording of pH values in effluents). The same applies to the food processing industry (recording pasteurisation temperature values).