answer to your question.
PD will be applicable if EB2,EB3 OR EB1 category is not current. PD is the date when you actually filed for labour which is the first step towards GC.
Assume that EB3 is not current, then they will mention the PD, anyone applied 485 whose PD is before or equal to the mentioned PD their case will be processed. People applied after the mentioned PD need to wait.Each center (ex..VSC, CSC..) will release the dates for which they are processing every month,so PD will be moving ahead every month depending upon the workload in the INS.
So PD plays a big role if the processing date is not current. If it is current all the three categories (EB1,EB2 AND EB3)are considered to be same.
Hope i have answered your question.
eddyja.