For what concerns the control system design, the dynamic behaviour of the plant has to be described by a mathematical model belonging to the class of the dynamic systems.

The plant description through such mathematical model is always only an approximation of the reality, which can neither perfectly nor completely be described through a mathematical model.

Therefore, because of the unavoidable approximation the model must be oriented toward the design of the specific control system one intends to implement, with the aim of grasping those aspects of the plant dynamics which are significant in view of the design to be carried out, and the knowledge of which is essential for reaching the established requirements.

As a consequence the mathematical model to be used for describing the dynamic behaviour of the plant will depend not only on the plant itself, but also on the performance requirements of the plant to be designed.

Given a plant and the control requirement specifications it does not and it can not exist (for what will be said later) any general rule defining how to build the corresponding mathematical model and enabling (even only in theory) to build it automatically. In fact any rule should have as a prerequisite the knowledge of the plant to be modelled, therefore it could be only approximate, so in order to be useful it should be oriented toward the model objectives, that is toward the design of the specified control system. Consequently it should be  necessary to start from a plant model which must – as an hypothesis – include with sufficient accuracy all information needed for the control design and for that reason gives an exhaustive description of all above mentioned aspects of interest. In other words for building the model required by the control design one should start from a model given a priori which - by hypothesis – should be fit for the control design: to prove the thesis one should assume the hypothesis that the thesis be true! Then the above general rule can not exist.

In conclusion, it is the designer who – on the grounds of his experience and knowledge of the plant, which is not necessarily transferred into a mathematical model – decides at his own risk that a given dynamic model defined by a finite set of parameters  (whose values might still have to be estimated) is fit for being taken as the foundation of a control system design aiming at the pre-established performance.