A foreign national serving in a capacity required for normal operations and service on board a vessel or aircraft. Crewmen are admitted for twenty-nine days, with no extensions. Two categories of crewmen are defined in the INA: D1, departing from the United States with the vessel or aircraft on which he arrived or some other vessel or aircraft; and D2, departing from Guam with the vessel on which he arrived.
Source: U.S. Citizenship and Immigration Services