Modern complex systems are normally constructed by interconnecting sub-units.

Therefore the systems engineer is often not too concerned about the details of what is inside these sub-units, provided it does what it says it will do 'on the tin'.

The systems engineer is primarily concerned with what happens when these sub-units are joined together.

To enable this, the sub-units are normally represented by 'Equivalent Circuits' with the appropriate parameters given by the manufacturer of the sub-unit as part of its specification data sheet.