From OpenSCADAWiki
Certain types of data sources themselves can generate data as they generate them entirely, as well as processing physical data, and even fully implement the gathering of these data into OpenSCADA environment and its internal language. Such data sources are called logical. Fully logical data sources are represented by modules: LogicLev and BlockCalc. There are several modules that combine logical data as a result of direct physical ones processing: ModBus, Siemens and OPC-UA.