The idea of clean separation of one part from another is so deeply ingrained that we have confidence that we can always separate inside from outside, even though it may take much mental effort. By analogy, we apply this concept...
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Drawing a system boundary around the area to be studied is not only valid in engineering, but also in business.
The most common boundary is that of the organization, which is represented by the company's profit/loss; but the boundary could be drawn around companies from the raw material to the final retailer, which would represent the profit/loss of the supply chain; or, alternatively, the boundary could be drawn around the individual production process within the company to determine the product's profit/loss.