Originally Posted by AlmostThere

"Advance Parole"(AP) is used for cases where your Adjustment of Status(AOS aka Green Card application) is pending.

Leaving the US without having AP, while AOS is pending, can deemed to be abandonement of the process. Therefore while the application is working its way through the system, you need to have this to leave US soil(which does not include American Samoa or Swains Island).

IANAL I: However once AP is used, technically your situation changes WRT to adjustment. If AP isn't used, you and your legal self are one. When AP is used your legal self remains at the border until your case is adjuticated. At which your legal self and you and unified again.

IANAL II: however there are certain cases, where advance parole IS NOT advisable. For example, if you have accumulated more than 180 days in the US without a legal status, and you leave the US, you can be barred from returning for a number of years specified by statute.(CIS will happily issue the AP document, without regards to these situation, so it user beware). Once your case is adjucated and you have a Green Card, this no longer applies.