8 CFR § 223.1 - Purpose of documents.
(a) Reentry permit. A reentry permit allows a permanent resident to apply for admission to the United States upon return from abroad during the period of the permit's validity without the necessity of obtaining a returning resident visa.
(b) Refugee travel document. A refugee travel document is issued pursuant to this part and article 28 of the United Nations Convention of July 29, 1951, for the purpose of travel. Except as provided in § 223.3(d)(2)(i), a person who holds refugee status pursuant to section 207 of the Act, or asylum status pursuant to section 208 of the Act, must have a refugee travel document to return to the United States after temporary travel abroad unless he or she is in possession of a valid advance parole document.