A model may be pictorial, descriptive, qualitative, or generally approximate in nature, or it may be mathematical and quantitative in nature and reas… - Harold Chestnut

Models are used essentially for evaluation and prediction purposes as well as for the analysis and study of the different parts of the system so that the systems engineer or designer may arrive at sound engineering decisions regarding the system design.

I should like to point out two other fields for serious attention by control people. These are (1) The need for 'optimizing the process of making automatic control', i.e. bridging the gap between theory and practice. (2) The need for working with qualified people in the social, economic, and political fields to help make the net effect of automatic control and automation a cause for hope rather than a reason for fear... The opportunities for a better world at peace make the challenge for using automation for the betterment of man one that is certainly worth working for.

The term closed loop-learning process refers to the idea that one learns by determining what s desired and comparing what is actually taking place as measured at the process and feedback for comparison. The difference between what is desired and what is taking place provides an error indication which is used to develop a signal to the process being controlled.