a living system, a man-made system or some kind of hybrid. So far, how- ever, SM&AML has no specific qualities for modelling living systems. Miller also talks about "closed systems", which are systems that do not interact with their environment (also sometimes called "esoteric box"). We will disregard these systems in this book. We can therefore reserve the concept "open system" to the meaning it has in standards: an open system is a system that communicates with systems in its environment according to standardized protocols and interfaces (to some extent). A concrete system is a "real thing". We will say that a concrete system is an instantiation of some kind of template that describes a possibly infinite number of similar concrete systems. In systems modelling we do not deal with concrete sys- items, but with information models of such systems. Except for living systems, Miller does not deal with domain-specific systems. In SM&AML, however, we make a basic distinction between two system domains, information systems and processing systems. 1.3 System Abstractions Human Human M/E/I flows Information Processing system system Information Data base system M/E/I flows Machine Machine concrete logical systems system Fig. 1.3 Information systems and processing systems Both models in Fig. 1.3 represent concrete systems, with different goals: The goal of an information system is to store data until somebody requests it or removes it. The time a datum is stored can range from days to many years. The processing that is done by an information system modifies the database on users' requests, and responds to questions (inputs). The output of an information system is information, based on stored data. The users of an information system are often humans, but can be machines as well. A processing system has some other (domain-specific) goal than to provide information. Its input and output flows can be any kind of matter, energy or information flow (M/E/I). For example, a fabric dyeing processing system takes matter in the form 