From my mono days I can tell you that the NOOD (National Offshore One Design) runs straight up boat for boat. OD classes get their own start. PHRF boats are lumped to gether classes based on number. We raced a boat with a PHRF of 78 and were lumped with boats ranging from 65 to 84 in what was called the Level 70 class. The sad thing is we beat these boats often times heads up regardless of rating.