STEP methods, the COMPUTATIONS must be done upstream of subcritical flow. This is done, as in subcritical flow the depth on the upstream side is high and on the DOWNSTREAM side it is low, hence stability is more on the upstream side.Similarly, for a supercritical flow, the step METHOD computations are done on the downstream side as the depth is more hence stability is more on the downstream side.The step method analysis requires a CERTAIN amount of time hence high velocity and low depth flows cannot be analyzed through it.