what is PD?

guys,I am confused. What is the date listed in the visa bulletin? Is that referring to the date that I applied LC or what? I have an RIR LC case pending in the infamous PBEC (thanks to my lawyer who suggest me not to wait on PERM). Now I was told better to sit my butt patiently to get out the PBEC mess because I can file 140 and 485 concurrently afterwards. Is that so?? thanks