Policy(1) is neither a stock nor a flow. Accumulations and flows of physical or abstract things, through operational thinking, are connected and modeled as stocks and flows in computer software. In addition, they are connected using information arrows indicating the direction of information. When policies are operationalized in a model they are elements (variables) placed between the information arrows connecting the stocks and flows creating a feedback loop. So there are two types of connections, one is the physical or abstract flow of things and the other is the information arrow creating feedback loops.
Policies in the real world are operationalized using laws, regulations, rules and other procedures. In an SD model, policies are operationalized using constants and variables input as elements between the arrows connecting the stocks and flows creating feedback loops.
Similar to human policies, the laws of physics are operationalized as equations in SD models to create feedback loops. The sciences have laws or rules that can be operationalized in an SD model using feedback loops.
For example, a stock with an outflow has a material flow. However if the level of the stock needs to regulate the rate of outflow then an information arrow connects the stock to the outflow so that level information is provided to the the outflow equation. This is a feedback loop. As the level of the stock changes the rate of the outflow changes causing the level of the stock to change, and the arrows make this connection continuous. In the real world this is called a bathtub with an open drain.
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