The design pattern originates from the seventies and enjoys accumulating interest nowadays. Maybe we do understand it better today.
Basically we approach everything as being a "system": it does something, consumes "signals" from the outside world and produces something. Another way to formulate IPO.
This pattern is quite useful when we consider the design of Information Systems. See it in action at http://enklare.wordpress.com/2011/02/28/the-b/
Such "systems" can be subdivided into sub-systems, which is done or expressed horizontally. A vertical sub-diviside is thought as aspect-systems and expressed vertically. So would a system consist of a financial ledger, sales ledger and purchasing ledger (as sub-systems) and the aspect "invoicing" would constitute a aspect-system. The distinguishing property is that sub-systems can live as-is, but a aspect-system cannot.
A system has its functionality in its "environment" or "context". Its outcome is measured and feedback is used to control the incoming triggers from the very environment. Sometimes the transformation process is adapted too.
With the Systems Approach, a company is really an interconnected group of systems that all work together. And the level of desired integration would distinguish these groups as individual sub-systems or systems at large.
The theory on Systems, cybernetics, considers also a hierarchical type of classification of systems. A system is said to be purposeful when, by their choices, they can produce (a) the same outcome in different ways in the same environment and (b) different outcomes in the same and different environments.
The beauty of this is, while a system may incorporate a lower type of system and may be incorporated in a higher type of system, that both the lowest type and the highest type do not exhibit choice of its own, see http://www.acasa.upenn.edu/4sys.htm