hope this helps...
ND is the date, what INS puts in the computer system as form received on and henceforth, when you check status of your I485 the AVM will say..INS got the form on this(ND) date and will take so & so days to process this kind of case..and they go ..blah..blah..blah.
Where as RD is when your mail reached them and filed(data not entered in system). This date your lawyer would know.