PD, RD & ND

1. Could you please help to clarify PD, RD & ND?

2. Do the online service center process dates (Receipt Notice Date) mean the ND?
(https://egov.immigration.gov/cris/jsps/Processtimes.jsp?SeviceCenter=Vermont)

3. What's the difference between RD & ND? What's the use of RD?

Thanks.