Advance Parole Document
Advance Parole (Form I-512) for adjustment applicants will be valid for a period which coincides with the time normally required for completion of an adjustment application not to exceed one year. (It means that it is valid for one year looking at the current status of adjustment of status processing.) It is usually valid for multiple entries. One copy for the applicant, one copy for the USCIS at the port of entry and one for the airline (so that they let you board the plane without other valid visa). First time you travel back to the US, you give the appropriate copies to the airline and USCIS, and USCIS will stamp your copy. Subsequent times, you can just show your copy and travel.

Form I-94 should be endorsed with the parole stamp. If Form I-512 indicates no specific authorization date, the parole should be valid for a maximum period of 1 year from the date parole was granted at the Port of Entry. Generally, for the I-485-based AP, the person is paroled in "indefinitely" for I-485 based advance parole.

Advance parole may be accepted by an airline instead of a visa as an authorization for the holder to travel to the US. It can't be issued to serve in place of any required passport. It can't be used to circumvent the normal visa issuing process neither is it a means to bypass delays in visa issuance.

Advance parole document earlier used to be a simple piece of paper which may be easier to counterfeit. Sample

Since Nov 24, 2003, USCIS redesigned the travel document and it incorporates increased security features aimed at eliminating a counterfeiter's ability to duplicate them, using the latest state-of-the-art technology. It adds security features that prevent the illegal production of these documents and improves production techniques and processes to shorten the processing times. Produced at the USCIS Nebraska Service Center, it utilizes the same patented personalization process as the current U.S. passport. This includes a digitized integrated photo, which has proven difficult for counterfeiters to duplicate. It also features a number of covert or hidden features that require sophisticated forensic equipment to view. It is light green in color and resembles the size and shape of the U.S. passport. Refugee travel documents and re-entry permits currently in circulation will remain valid until the expiration dates printed on those documents. The use of new production equipment and techniques has increased the production rate by ten fold of the previous production rate.

Since Oct 25, 2011, those who have applied for EAD and Advance Parole together will get the Advance Parole combined as part of the EAD document.

Mere possession of advance parole does not guarantee admission into the U.S. as the alien is still subject to immigration inspection at a port of entry to determine admissibility into the U.S.. There also may be bars as described below. You may be subject to "secondary inspection" where you will be taken to a separate room and asked questions.

Travel Warning
If you have been unlawfully present in the US for more than 180 days but less than 1 year and you leave before removal proceedings are started against you, you may be inadmissible for 3 years from the date of departure.

If you have been unlawfully present in the US for 1 year or more, you leave regardless of before, during or after removal proceedings, are started against you, you may be inadmissible for 10 years from the date of departure.

Only unlawful presence that was accrued on or after Apr 1, 1997 counts towards the 3 and 10 year bar.

Certain immigration benefits and time spent in the US while certain applications are pending may place you in a period of authorized stay. e.g., a properly filed application for adjustment of status, Temporary Protected Status (TPS), deferred enforced departure (DED), asylum and withholding of removal. Bars to enter the US, as described above, may still be triggered if you accrued more than 180 days of unlawful presence BEFORE the date you were considered to be in a period of authorized stay. For refugees and asylees, the waiver may be granted for humanitarian reasons, to assure family unity or if it is otherwise in the public interest.

Waiver of inadmissibility on humanitarian can generally be applied only by those persons that can establish extreme hardship to their US citizen or lawful permanent resident spouse or parents, to assure family unity or when it is otherwise in public interest.

Invalidation
Travel documents will be invalid if you obtained them by making a material false representation or concealment in the application, or if you are ordered removed or deported from the U.S.