System Specification for IEC 61850 is an optional component and is used to specify from a high level "what" the system should look like and "what" functions it should perform. It allows a standards based approach to design that provides a framework for creating repeatable and reusable engineering. It is not interested so much in the "how" a system is implemented- albeit the how will often be in mind during this phase to some extent. Given that the "what" is specified without the "how" it is the enabler of IEC 61850's desire for interoperability and interchangeability. The "Top down engineering" approach in System Engineering begins with the creation of a comprehensive System Specification. Thorough detail in the System Specification stage reduces engineering time and can simplify the later "System Integration" Stage.