Systems in the Functional Domain
From the development in Sec. C4.3, we understand that the real functional elements associated with a plant describe the functionality of the plant in varying detail; they are related in a hierarchical fashion, but there is no interaction between real functional elements associated with a given plant. We also saw that, in order to demonstrate that the functionality of a plant satisfies the stakeholder requirements, we need two types of imaginary functional elements; primary and secondary elements, and describe the real functional elements as systems of imaginary elements. So, as far as a given plant is concerned, interactions in the functional domain are between imaginary elements only, and this immediately raises a few questions: Are interactions between secondary elements identical to interactions between primary elements? What is the nature of interactions between primary and secondary elements? Do the primary and secondary elements form discernable subsystems? And if so, is there any benefit, in terms of better insight and understanding, in viewing them as subsystems?
Before going further, I should make a brief comment regarding the use above of the qualifier “associated with a given plant”. It reflects the meaning of the term “system of interest”  and the fact that the system concept implies a boundary; a rule that divides the universe of elements into those that belong to the system and those that do not. If we combine two systems, A and B, to form a “system of systems”, then, within the context of this new “system of interest”, the real elements representing the systems A and B in the functional domain disappear, and the new system is represented by a real element describing the functionality of that system. That real element will be a particular system of some or all of the imaginary elements making up the two systems A and B. In other words, real elements are defined relative to “systems of interest”, and at any one time there is only one “system of interest”.
KeywordsFunctional Domain Functional Parameter Emergent Property Functional Element Primary Element
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