If you are in the U.S. and have applied for a green card but are still waiting for your application to be processed, then you might be interested in applying for a travel permit to travel outside the U.S.. Otherwise, if you leave without the travel document, USCIS may consider your green card application abandoned. If you already have a green card, and wish to leave the U.S. for more than a year but less than two years, then you might also be interested in arranging for a travel document before you leave.