The processes are typically modeled as resource flows, the constraints typically represent limits on flows. In his book "The Goal", the protagonist is a manager in charge of a troubled manufacturing operation. At any point in time, one particular constraint (such as inadequate capacity at a machine tool) limits total system throughput, and when the constraint is resolved, another constraint becomes the critical one. The plot of Goldratt's stories revolve around identifying the current limiting constraint and raising it, which is followed by finding out which is the next limiting constraint. Another common theme is that the system being analyzed has excess capacity at a number of non-critical points, which, contrary to conventional wisdom, is absolutely essential to ensure constant operation of the constrained resource.