Answer: At the beginning of Thermoptim, the only components available in the software were single-function. These components are so-called single-function because they are related either to a mechanical energy, either to a thermal energy. For instance, the following devices are called single-function: compressors, nozzles for adiabatic expansion (by far the most known and widespread), nozzles for isenthalpic expansion, heat-exchangers, and combustion chambers. Multi-functional components are more sophisticated. They can handle several thermal couplings at various temperature levels. In the previous versions of the software, it was not possible to describe such components.