In the branch concerned with work in the systems sciences as such, we can distinguish between the purely theoretical development of systems ideas and their interrelationships, and work aiming to develop systems ideas useful to interpreting and/or handling real-world situations.
General evolution theory is an example of the former, while the development of social systems design methodology is an example of the latter. There are others examples as well, leading to a three-fold distinction: hard systems approaches (such as are employed in systems engineering), soft systems approaches (such as are drawn upon in humanistic psychology), and mixed systems approaches — such as those employed in operations research — used as an aid to
decision-making.