While your application for Adjustment of Status (AOD), Form I-485, is pending, you must not leave the United States for any reason until you have received permission to reenter the United States in the form of advance parole. Failure to do so will result in abandonment of your green card application.
Advance parole (AP) allows certain foreign nationals to reenter the United States after temporary travel abroad without the need of any visa. AP is almost like a visa for entry into the US, although you still need a foreign passport to along with your AP document.
Foreign nationals who have a pending application for permanent residency or for change in nonimmigrant status require AP to re-enter the United States after traveling abroad.
If you leave the United States, without first obtaining AP, while your Adjustment of Status (AOS) is pending, then you are considered to have abandoned your application and will not be allowed to enter the United States. In such cases, the person will have to go through consular processing.
However, if you are in valid nonimmigrant status, such as H or L status, then you can travel on the basis of that immigration status even while your AOS is pending and without AP.
It is valid for one year during which time the foreign national may make multiple entries into the US. Furthermore, the AP may be renewed as long as the renewal application is filed 120 days before expiration of the previously issued AP document.
The AP application usually takes about two to three months to process.
NOTE: if you have been out of status for more than 180 days, then you may be subject to the 3/10-year immigration bar if you travel abroad, even if your AP application is approved.
A reentry permit is issued to a permanent resident who plans on remaining outside the US for more than one year but less than two, and allows then to reenter the United States after such travel abroad. Failure to obtain a reentry permit prior to travel outside the United States will likely lead USCIS to conclude that the residency has been abandoned.
The reentry permit is akin to a passport for the permanent resident, in case he or she cannot obtain a passport from his or her country of nationality.
An important point with regard to those who intend to travel for more than 180 days on a reentry permit is that such an absence will interrupt the five-year continuous presence requirement that a person must fulfill to qualify for citizenship.
It is valid for 2 years. They cannot be renewed and the person can only apply for it while inside the United States.
Note also that if the foreign national stays outside the US for more than one year without a reentry permit, then he or she will have to apply at a US Consulate abroad for a special immigrant visa as a returning resident. Obtaining the visa will require the applicant to convince the consular officer that the absence from the US was temporary and due to unforeseen circumstances, and that you never planned on abandoning your US residence.