Introduction

The goal of systems dynamics approach to modeling complex systems is to describe the complexities using stock, flows and feedback loops. Not all inputs are flows, for example a constant perturbation in an otherwise decaying system is considered to have only one flow that is the net diffrence between the constant and decay term i.e. a goal seeking dynamics where the systems chases the goal and once reached stays there (a stable equilibrium). 

dE/dt=(E0-E)/RAT

E0 is the goal (equilibrium) E is the expected sales stock. There is only an inflow that adjusts to the goal, there are no inflow and outflow. A stock flow diagram is shown in Figure 1.

Other basic dynamical behaviors such as exponential growths (uncontrolled growth), goal seeking dynamics, S-shaped (growth with constrains), Oscillatory, growth with overshoot, overshoot and collapse  together with time delays form the building blocks in many complex systems ranging from service industry to population growth.