Most web resources about Requirements Specifications are not very relevant to those we need to produce for process control systems.
Generally although the two share some of the same terminology the objectives and methodologies are different.
Not least, this is because process control has existed since long before IT - or even computers.
The IT world generally uses the Unified Modeling Language (UML) for specifying, visualizing etc - but UML was never designed for Process Control.
In fact Process Control specifiers have used diagrams of various types (that is really a form of modeling) for many decades and Process Control has existed for much longer.
The History of UML
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