Confused about how priority dates fit in...

[ETA-GC]

Hi all.

After reading the info on priority dates (PDs) at

http://boards.immigrationportal.com/showthread.php?t=143061&page=2&pp=15, http://boards.immigrationportal.com/showthread.php?t=143292 and http://boards.immigrationportal.com/showthread.php?t=137755 I'm now somewhat confused about how these are used.

Does one's PD merely determine your place in line for a GC visa number or does it also affect the order in which cases are processed?

For example, Sam and Max both filed their I-140 / I-485 on the same day, but had Sam started his Labor Certification (LC) much earlier (Sam was located in a state where LC is slow and his lawyer sat on his case for 6 months before filing the GC after the LC was approved).

So Sam has a much earlier PD than Max (maybe as much as 1 year earlier). How does this afffect Sam? Assuming their cases / situations are otherwise identical, should Sam get processed before Max or does the PD not affect the order in which the I-485's are adjucated?

ETA

 

[usnycus]

From my understanding PD is to get in line for GC.

In hypothetical scenario of Sam & Max, anyone can get GC approved first as long as PD remains current for both.

PD comes in picture if it's not current. In that case, individual have to wait to file I-485 if it's not already done. If they have already applied for I-485 then person with PD later than current date will wait and in the mean time s/he can stay in US.