It is self-evident that the pilots of an ocean liner and a super-sonic airplane need to have different personalities because the natures and the characteristics of their vehicles are different. This is also true in industrial process control. The pilot (the controller) must be correctly matched to the process that it controls. In order to do that, the designer of a process must understand the ‘personality’ of the process.
Most processes contain resistance, capacitance, and dead-time elements, which determine their dynamic and steady-state responses to upsets. Before the control of processes is discussed, these three elements of the ‘personalities’ of processes will be described.
To describe the personalities of processes, block diagrams are used. The two main symbols used in any block diagram are a circle and a rectangular box. The circle represents algebraic functions such as addition or subtraction and is always entered by two lines but exited by only one.
The rectangular box always represents a dynamic function, such as a controller, where the output is the controlled variable (c) and is a function of both time and of the input or manipulated variable (m). Only one line may enter and only one line may leave a rectangular block. The ‘system function’, the ‘personality’ of the process component, is placed inside the block, and the output is determined by product of the system function and the input.
[Liptak, B, Process Control and Optimization, Instrument Engineers Handbook, 4th Edn, CRC Press, Florida, pp. 97]

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