This article is from the FAQ, by with numerous contributions by others.
All immigrant applicants (whether applying at INS offices or abroad)
must have priority dates earlier than any applicable visa cut-off
dates to be granted a visa or adjustment of status. This has not
been altered by the recent amendment to the Immigration Act. Under the
terms of Section 245, INS will not accept an application for adjustment
of status from an applicant in a numerically limited category unless
a visa number is available for the alien's priority date. All immigrant
visa applicants will continue to be processed in order of priority date
whether cases are processed at overseas American Consulates or through INS.