Stepper motor driver design can be either Unipolar or Bipolar. Without getting into circuit design details, from the standpoint of the user, bipolar wiring requires a much more complicated (and expensive) driver and does not allow microstepping. Sherline made the engineering decision to design our driver box around a unipolar layout mainly because it allows half stepping (micro-stepping) which provides a very fine 400 steps/revolution resolution. We specified a sufficiently powerful stepper motor to begin with so that you have plenty of power using this more economical, high resolution design. To learn much more about about the differences between the two wiring schemes see http://eio.com/jasstep.htm#types.