In addition, U.S. law also limits the number of visas available in certain categories by country. For limited categories, the availability of immigrant visa numbers depends on the date your petition was filed. This is called your priority date.
https://travel.state.gov/content/visas/en/immigrate/immigrant-process/approved.html
Permanent Residence Process and Priority Dates. In order to obtain employment based permanent residence, in most cases, there are three steps. The following is an outline of the process. If you have follow up questions, please do not hesitate to contact us at 305-895-0300.
http://www.visaattorneys.com/visas/immigrant-visas/immigrant-employment-based-visa
When your petition becomes current, or is likely to become current within one year, the NVC initiates immigrant visa pre-processing, including collecting visa fees, forms, and documents from sponsors (petitioners) and immigrant visa applicant(s).
https://travel.state.gov/content/visas/en/immigrate/immigrant-process/approved.html
Once your priority date is current, we may assist by preparing and filing the many forms for the I-485 on behalf of the employee. Approximately 90 days after filing, the employee will receive an Employment Authorization Document (EAD Card), which will permit him/her to work in the US while the I-485 is pending.
http://www.visaattorneys.com/visas/immigrant-visas/immigrant-employment-based-visa
After Your Petition is Approved. Petitions (Forms I-130 and Forms I-140) approved by USCIS in the United States are sent to the Department of State's National Visa Center (NVC) for pre-processing.
https://travel.state.gov/content/visas/en/immigrate/immigrant-process/approved.html
The priority date is set as the date your PERM Labor Certification application is filed with the Department of Labor. Priority dates can move several months ahead in only thirty days. We have also seen the priority date move several months backwards.
http://www.visaattorneys.com/visas/immigrant-visas/immigrant-employment-based-visa
Visa Bulletin. 1  USCIS, in coordination with Department of State (DOS), is revising the procedures for determining visa availability for applicants waiting to file for employment-based or family-sponsored preference adjustment of status.
https://travel.state.gov/content/visas/en/law-and-policy/bulletin.html
An individual who is the beneficiary of an approved immigrant petition and has an immigrant visa number immediately available may apply at a U.S. Department of State consulate abroad for an immigrant visa in order to come to the United States and be admitted as a permanent resident.
https://my.uscis.gov/helpcenter/search?q=Priority+Dates&tag=tag_search
The priority date is the earliest date to which a patent application can claim priority. The priority date defines what materials are prior art to the patent application; thus, obtaining the earliest possible priority date can be important to the patentability of some inventions. In some cases, the filing date sets the priority date.
https://www.richardspatentlaw.com/faq/what-is-a-priority-date/
Please refer to the Visa Bulletins above for final action dates established during the current fiscal year. Please refer to the Visa Bulletins above for specific Employment Fourth (E4) Preference final action dates for El Salvador, Guatemala, and Honduras (beginning with the May 2016 Visa Bulletin).
https://travel.state.gov/content/visas/en/law-and-policy/bulletin.html
what is a priority date
A priority date is the earliest date to which a patent application can claim priority.