When you apply to become a lawful permanent resident, if you travel outside the US, your case is considered abandoned. One easy way to get permission is to file for Advanced Parole (I-131) when filing your Green Card application. Basically, this gives you advanced permission to reenter the country before you even leave.This does not require an extra filing fee, and is a good idea to file with your Green Card application in case you will have to leave for any reason. This form comes with your I-765 work card.
We had a client once who had no other means of entering the US and her case was abandoned because she had to leave last minute to care for her ailing father in another country, USCIS contacted us to let us know the case had been abandoned and she lost her filing fee AND her place in line.
This can also revoke B-1 and B-2 or other non-immigrant visas in the future because you have demonstrated immigrant intent, meaning you want to live here and plant your roots.
Basically, if you file an I-45, you should file an I-131 along with it just in case.
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