This article forms part of the Systems Fundamentals Knowledge Area (KA). It provides various perspectives on system classifications and types of systems, expanded from the definitions presented in What is a System?.
The modern world has numerous kinds of systems that influence daily life. Some examples include transport systems; solar systems; telephone systems; the Dewey Decimal System; weapons systems; ecological systems; space systems; etc. Indeed, it seems there is almost no end to the use of the word “system” in today’s society. This article considers the different classification systems which some Systems Science (glossary) authors have proposed in an attempt to extract some general principles from these multiple occurrences. These classification schemes look at either the kinds of elements from which the system is composed or its reason for existing. The idea of an engineered system (glossary) is expanded. Four specific types of engineered system context are generally recognized in systems engineering: product system, service system, enterprise system and system of systems capability.
System Classification
A taxonomy is "a classification into ordered categories" (Dictionary.com 2011). Taxonomies are useful ways of organizing large numbers of individual items so their similarities and differences are apparent. No single standard classification system exists, though several attempts have been made to produce an useful classification taxonomy. Bertalanffy (1968) divided systems into nine types, including control mechanisms, socio-cultural systems, open systems, and static structures. Miller (Miller 1986) offered cells, organization, and society among his eight nested hierarchical living systems levels, with twenty critical subsystems at each level. Peter Checkland (Checkland 1999) divides systems into five classes: natural systems, designed physical systems, designed abstract systems, human activity systems and transcendental systems. Checkland refers to these five systems as comprising a “systems map of the universe”.
Systems of Systems
Systems can be grouped together to create more complex systems. In some cases it is sufficient to consider these systems as systems elements in a higher level system, as part of a system hierarchy. However, there are cases where the groupings of system produce an entity that must be treated differently from an integrated system. The most common groupings of systems that have characteristics beyond a single integrated system are Systems of Systems (SoS) and Federations of Systems (FoS). Maier examined the meaning of System of Systems in detail and used a characterization approach which emphasizes the independent nature of the system element, rather than “the commonly cited characteristics of systems-of-systems (complexity of the component systems and geographic distribution) which are not the appropriate taxonomic classifiers” (Maier 1998, 268). Wherever independent systems are combined into groups the interaction between the systems adds a further complexity; specifically, by constraining how the resulting system can be changed or controlled. This dimension of complexity affects the management and control aspects of the systems approach.
Engineered Systems Classifications
The classification approaches discussed above have either been applied to all possible types of systems or have looked at how man-made systems differ from natural systems. A product or service is developed and supported by an individual, team, or enterprise. The nature of engineered systems has changed dramatically over the past several decades from systems dominated by hardware (mechanical and electrical) to systems dominated by software. In addition, systems that provide services, without delivering hardware or software, have become common as the need to obtain and use information has become greater. Recently, organizations have become sufficiently complex that the techniques that were demonstrated to work on hardware and software have been applied to the engineering of enterprises. Three specific types of engineered system context are generally recognized in systems engineering: product system, service system and enterprise system.
Products and Product Systems
The word product (glossary) is defined as "a thing produced by labor or effort; or anything produced" (Oxford English Dictionary). In a commercial sense a product is anything which is acquired, owned and used by an enterprise (hardware, software, information, personnel, an agreement or contract to provide something, etc.). Product systems (glossary) are systems in which products are developed and delivered to the Acquirer (glossary) for the use of internal or external user. For product systems, the ability to provide the necessary capability (glossary) must be defined in the specifications for the hardware and software or the integrated system that will be provided to the acquiring enterprise (glossary).
Services and Service Systems
A service (glossary) can be simply defined as an act of help or assistance, or as any outcome required by one or more users which can be defined in terms of outcomes and quality of service without detail to how it is provided (e.g., transport, communications, protection, data processing, etc.) Services are processes, performances, or experiences that one person or organization does for the benefit of another, such as custom tailoring a suit; cooking a dinner to order; driving a limousine; mounting a legal defense; setting a broken bone; teaching a class; or running a business’s information technology infrastructure and applications. A service system (glossary) is one that provides outcomes for a user without necessarily delivering hardware or software products to the service supplier. The hardware and software systems may be owned by a third party who is not responsible for the service. The use of service systems reduces or eliminates the need for acquirers to obtain capital equipment and software in order to obtain the capabilities needed to satisfy users.
Enterprises and Enterprise Systems
An enterprise (glossary) is one or more organizations or individuals sharing a definite mission, goals, and objectives to offer an output such as a product or service. An enterprise system (glossary) consists of a purposeful combination (network) of interdependent resources (e.g., people; processes; organizations; supporting technologies; and funding) that interact with 1) each other (e.g., to coordinate functions; share information; allocate funding; create workflows; and make decisions), and 2) their environment(s), to achieve business and operational goals through a complex web of interactions distributed across geography and time (Rebovich and White 2011, 4, 10, 34-35).
According to Maier’s definition, an enterprise would not necessarily be called a system of systems (SoS) (glossary) since the systems within the enterprise do not usually meet the criteria of operational and managerial independence. In fact, the whole purpose of an enterprise is to explicitly establish operational dependence between systems that the enterprise owns and/or operates in order to maximize the efficiency and effectiveness of the enterprise as a whole. Therefore, it is more proper to treat an enterprise system and an SoS as different types of things, with different properties and characteristics (DeRosa 2005). Enterprise systems are unique, compared to product and service systems, in that they are constantly evolving; they rarely have detailed configuration controlled requirements; they typically have the goal of providing shareholder value and customer satisfaction, which are constantly changing and are difficult to verify; and they exist in a context (or environment) that is ill-defined and constantly changing.
System of Systems Capability
The term System of Systems Capability is used here to describe an engineering context in which a number of enterprise, service and product systems are brought together dynamically to provide a capability which is beyond the scope of any individual enterprise. Understanding the need for system of systems capability is a way of setting a broader problem context for the engineering of other systems. Both product and service systems may be engineered to both satisfy immediate stakeholder needs and to have the potential to be used for the composition of SoS capabilities. Engineering at the Enterprise level can include an Enterprise Capability Management activity, in which possible SoS problems are explored and used to identify gaps in the enterprise's current product and service portfolio.