At each hierarchical level are distinguished mathematical models of components and systems models of systems, obtained by direct 
the union of mathematical models of elements in the general system of equations, called the complete mathematical model. 
Approximation of the full models are more simple in terms of computing time or memory in use, 
call macro

A characteristic feature of CAD objects of modern physical instrument making, including the EC and UZCH is shifting the center of gravity on the stage of engineering design.Indeed, on the one hand, this stage is the most labor-intensive and requires the mandatory use of powerful computational tools of the extreme complexity of the major subsystems and components. On the other hand, the tasks to be undertaken during the detailed design (automation of design work and documentation), are usually poor problem orientation, which allows you to successfully use the appropriate sub-systems and other domains. This, incidentally, corresponds to one of the basic principles of creation of any CAD system - the principle of invariance, which requires that the subsystems and components of the CAD system were possible, universal or model, ieinvariant to the projected targets and sector specific. Therefore, almost always under the EC and CAD UZCH involve automation of engineering design.