Answer: If you are not a U.S. Citizen, You may need permission to return to the United States after traveling abroad.
This permission is granted through a travel document.
Travel documents are also given to people who want to travel, but cannot get a passport from their country of nationality.

Answer: You must file USCIS Form I-131,
Application for a Travel Document, Complete with supporting documentation, photos and applicable fees.
If you are a permanent resident or conditional resident,
You must attach a copy of the alien registration receipt card;
Or if you have not yet received your alien registration receipt card.
A copy of the biographic page of your passport and the page of your passport indicating initial admission as a permanent resident, or other evidence that you are a permanent resident;
Or a copy of the approval notice of a separate application for replacement of the alien registration receipt card or temporary evidence of permanent resident status.

Answer: If you are in the United States,
You must show a copy of any document issued to you by the Service showing your present status in the United States;
An explanation or other evidence demonstrating the circumstances that warrant issuance of Advance Parole.
If you are basing your eligibility for Advance Parole on a separate application for adjustment of status or asylum,
You must also attach a copy of the filing receipt for that application.
If you are traveling to Canada to apply for an immigrant visa,
You must also attach a copy of the consular appointment.

Answer: Where to file the Form I-131 depends upon the benefit sought. Thus,There is not only one single place to file.

Answer: You must apply for the travel document before leaving the United States.
Failure to do so may cause you to lose permission to re-enter the country,
And lead to the denial of any other applications.