from
http://www.immihelp.com/gc/aos/ap.html
The approved Advanced Parole form is called I-512 and usually three copies of them are given. 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 with out other valid visa.) First time you travel, you give the appropriate copies to airline and USCIS, and USCIS will stamp your copy. Subsequent times, you can just show your copy and travel.