What is the Priority Date?
The priority date is the date the first paperwork for permanent residence is filed with a government agency. For an Employer-sponsored case, this is either the date a labor certification is first filed with the Department of Labor, or if not a labor certification-based case then the date the immigrant petition (most often an I-140 Immigrant Visa Petition) is first filed with USCIS. For a family-sponsored case, this will be the date the I-130 Petition for Alien Relative is filed (this may or may not be the same date as the I-485 is filed).
Why is the Priority Date important?
The Priority Date establishes the foreign national's place on line for an immigrant visa. There are limited numbers of immigrant visas available for each of the various categories. These categories are called "Preference Classes." Each preference class has its own queue for immigrant visas, and the priority date and the preference class of the petition determine how long the person being petitioned for has to wait for a green card.
What are Preference Classes?
Preference Classes are numbered categories which define types of immigrant visas. There are separate sets of preference classes for family-based immigration and employment-based immigration. The case must fit into one of the preference classes for an immigrant petition to be approvable, and the preference class into which it fits must be indicated on the form filed for that petition (the I-140 for employment-based petitions, the I-130 for family-based petitions). The best way to find out what the different preference classes are is to look at the Department of Status current Visa Bulletin (the explanation is always there): http://travel.state.gov/visa/bulletin/bulletin_1360.html
How are the Available Immigrant Visas Allocated Between the Preference Classes?
There are a certain number of immigrant visas allotted to each preference classification in both the family and employment categories. Within each preference classification, a certain number of immigrant visas are allotted to each country. Sometimes, countries with high levels of immigration to the U.S. become "oversubscribed" - more people "charged to" that country want immigrant visas than there are immigrant visas to go around - before other countries. The country to which a foreign national is "charged" or counted against is the foreign national's country of birth (not current citizenship or nationality). Countries which often become oversubscribed more often than most other countries include India, Mexico, the Philippines, and sometimes China. Availability for these countries is broken out separately by the U.S. State Department on its web site.
How do the Priority Date and Preference Class come together to determine how long someone must wait for a green card?
The act of filing the first petition or application for permanent residence (labor certification, I-140, or I-130) establishes the priority date. If you have ever been required to take a ticket to await service (i.e.: deli counter at the supermarket, or Motor Vehicle Department in your state), think of the act of filing a first petition or application as taking that numbered ticket. The date the petition or application is received by the agency where it was filed is your number, or place on line. Every month, the U.S. Department of State, which controls immigrant visa availability, updates its web site with the date which is now "current" for each employment- and family-based preference class. Think of this web page as the "now serving" display at the deli counter or state Department of Motor Vehicles. The date displayed for your preference class must be at your priority date (or more recent) before an immigrant visa is available, and the case can proceed.
Can you give an example of how this works in practice?
Say that a relative filed an I-130 Petition for Alien Relative on your behalf in the Second Preference Part "B" category, which was first received by the government on June 1, 1998. Let's say that when this case is approved, the State Department web page lists the date of May 1, 1998 for the Family-based Second Preference Part "B" category. In this instance, your priority date is not yet current but should be relatively soon. When the Second Preference Part "B" category is updated from May 1, 1998 to a date on or after June 1, 1998, you are "now being served" - you may now file an I-485 Application to Adjust Status to Permanent Residence if eligible (or process through a consulate abroad, if you can do this without becoming subject to the three or ten year bar).
How do backlogs in availability of Immigrant Visa numbers cause delays in how long it takes to get permanent residence?
When an immigrant visa number is immediately available, an Application to Adjust Status to Permanent Residence may be filed in the U.S., if eligible, or the foreign national may process through a consulate abroad. A delay is caused when an immigrant visa isn't immediately available for a petition of a given preference class for a person of a given country of birth at the time the immigrant visa is to be filed. Nothing further can be done until the immigrant visa becomes available, per the Department of State.
Can I ever can I just file both parts of the case together?
Yes .Filing of the I-485 Application to Adjust Status to Permanent Residence along with the petition for the I-140 Immigrant Visa Petition is permitted, but this is possible only where there is an immediately available immigrant visa number (usually the first preference categories, sometimes the second preference for certain countries of birth). Similarly, where an I-130 Petition for Alien Relative is filed for an immediate relative family-based case, the I-485 Application to Adjust Status to Permanent Residence may be filed simultaneously (but this is not true for other family-based cases).