It is sometimes the case that the flow into a stock is not dependent on the amount of stuff stored in that stock. A bathtub is often used as an example. Think of the water stored in the bathtub as a stock. Turn the spigot on and walk away and the flow into the bathtub is not determined by the stock. The same is true for river flow into Lake Erie -- the rivers have no capacity to adjust flow based on how much water is already in the Lake. Of course in both cases the stock has the capacity to overflow if teh stock exceeds some maximum capacity -- over the sides of tub or down Niagara Falls.