Box or budget models are a widely used type of model that conceptualize a system as being composed of stocks, flows, feedback connectors and auxiliary variables that control flows. Stocks: any system component that stores stuff. The “stuff” can be energy, materials, number of organisms, widgets, information, love or anything that can accumulate or be depleted over time. In modeling lingo stocks are also often referred to as state variables because at any moment in time the size of the stocks in a model is an indication of the state of the system.Flows: the movement of stuff into and out of stocks. Flows are also referred to as rate equations or differential equations. Connectors or links: arrows in diagram that connect stocks, flows and auxiliary variables to indicate dependencies or linkages.Auxiliary variables: An assortment of other constants (e.g. coefficients), equations, data, graphical relationships and odds and ends are necessary for modeling building. Different modeling software use all sorts of different terms to describe this assortment.