If you are not a U.S. citizen, you may need permission to return to the U.S. after traveling abroad. This permission is granted through a travel document. Travel documents are also given to foreign nationals who want to travel, but cannot get a passport from their country of nationality.
Advance Parole is permission for certain foreign nationals, who do not have a valid immigrant visa, to re-enter the U.S. after traveling abroad. Such foreign nationals must be approved for Advance Parole before leaving the U.S. If you have not obtained Advance Parole prior to traveling abroad, you will not be permitted to re-enter the U.S. upon your return.