Priority Date, the time when you filed AEC.
So your PD is 09/2000. in the past their is a "cut-off date" for each EB category/Country, based on quota assigned by law. It is called in Chinese community "Pai2-Qi1" (cut-off dates) or "Pei4-E2" (quota). You can\'t file I-485 until your PD is the same or later than your cut-off date. Fortunately, according to Bill 2045 (passed in Oct/2000), the cut-off dates were moving so fast that when your I-140 got approved you didn\'t have to wait! That\'s why you don\'t even understand our questions about PD.