Immigrants in the United States that have started the process of putting in an application for certain visas or a green card should not leave the country during the time that their application is being processed. People who have not completed all paperwork might find themselves unable to re-enter the country, so it is crucial to apply for the right documentation.
There are sometimes emergency situations that force an immigrant into traveling while they are still waiting for their visa or green card application to be accepted. Under these circumstances they would need to fill out an advance parole document so that they are able to make short international trips. This document, also often referred to simply as a travel document, is not required to leave the United States but when the immigrant returns border authorities might assume that they have abandoned their application for a visa or green card and refuse them to allow re-entry.
It is also good advice for an immigrant to fill out the travel document at the same time as they put in their application for a green card. The processing of applications for green cards can take a very long time (between nine to 12 months) and the safety of that application is strengthened by completing the advance parole document.
There are a number of different reasons why an applicant might want to leave the United States while they are still waiting to get a green card, such as business trips, family emergencies, weddings and funerals.