If you are in the United States and are a US citizen, you can travel freely outside the US for extended periods of time and still retain your citizenship. You can travel as often and as long as you like with your US passport. However, only US citizens have US passports, which means that most other immigrants in the US need some form of travel document in order to travel outside of the United States.
US green card holders are free to travel in and out of the United States as often as they wish, so long as they do not stay outside the United States for one year or more. Travelers who will be outside the US for that period of time may not be readmitted to the US unless they have secured a travel document known as the re-entry permit before leaving the United States. Failure to do so can mean that a US green card holder will lose status and will not be allowed back into the United States. Travelers with a US green card who may be outside of the US for a year or more need to file USCIS Form I-131 to secure a re-entry permit. They must have this travel document in hand before leaving the United States.
US green card holders who have been approved for a green card but who have not yet received their green card in the mail must also apply for a re-entry permit by filing Form I-131. These travelers must apply for a travel document even if they are only leaving the United States very briefly for an emergency.
There is also another form of travel document known as Advance Parole. If you have applied for immigration benefits or a change of status but need to travel before you hear back about your petition, you must apply for Advance Parole before you travel. Failure to do so means that your application for immigration benefits of change of status may be considered abandoned. You may have a hard time re-entering the US without an Advance Parole document and even if you do re-enter you will need to apply all over again. You can apply for Advance Parole by filing Form I-131.
Refugee and asylum status holders in the United States must also apply for a travel document – known as a refugee travel document – before leaving the United States. Eligible applicants can apply for this travel document by filing Form I-131 as well. It is important to have a refugee travel document, if you need it, before leaving the United States. Failure to do so can mean lass of asylum or refugee status and denial of entry into the United States.