It means that if your priority date is before this date and you will apply for the immigrant visa in China (not filing I-485) to get your green card, probably NVC will contact you and ask you to provide the related documents, but you cannot file I-485 within US because USCIS required that the I-485 filing is based on the Final Action Date, not the Filing Date in July 2017. See:
https://www.uscis.gov/green-card/when-file-your-adjustment-status-application-family-sponsored-or-employment-based-preference-visas-august-2017
So, you need to the above link every month to see whether the I-485 filing is based on the Final Action Date or the Filing Date to determine whether you are qualified for I-485 fling in that month.
https://www.uscis.gov/green-card/when-file-your-adjustment-status-application-family-sponsored-or-employment-based-preference-visas-august-2017
So, you need to the above link every month to see whether the I-485 filing is based on the Final Action Date or the Filing Date to determine whether you are qualified for I-485 fling in that month.