from murthy.com
Question 4. What is a travel document and do I need it? TOP
A travel document (Advance Parole) permits a person to travel outside of the U.S. while the adjustment application is pending. It is necessary if one needs to travel during the pendency of the adjustment application and does not have another status on which to reenter the U.S., namely H1B or H-4, L-1 or L-2. The earlier doctrine, that leaving the U.S. while the adjustment application is pending could be considered abandonment of the application unless advance parole is received prior to traveling, is no longer valid for Hs and Ls. Since June 1999, one can travel and reenter the U.S. on H or L status by showing both the visa stamp in the passport and the petition approval, no longer requiring one to file an Advance Parole. For a person in any other status (besides H1B or H-4, L-1 or L-2), leaving the U.S. while the adjustment application is pending would still be considered an abandonment of the application, unless advance parole is received prior to traveling.
Please note that a person who is out of status may be advised not to travel abroad until the Adjustment is approved. One who has overstayed 180 days and then travels abroad would not be readmitted for 3 years, while for a one-year overstay there is a 10-year bar on reentry. The advance parole document would not protect one in this situation and, in fact, the document since 2000 contains a specific warning about the 3-year and 10-year bars. You should seek the specific advice of an attorney on that point if there is a possibility this applies to you.