An itinerary containing an ARNK (arrival unknown, pronounced 'arunk') segment is one in which the traveler arrives at destination A but departs out of destination B. The arrival to destination B is unknown, hence the name arrival unknown [1]. It is similar to an open-jaw ticket except that there is no variation in the point of origin. The reason for this is that airline reservation systems (and major GDS') have to have the segments following on sequentially, so arriving at one city, and then departing from another, will cause the system to throw up an error message. The ARNK field tells the system that this is intentional.
An example of an itinerary that contains an ARNK segment is the following:
Segment 1: 11-NOV: SFO/IAD (San Francisco to Washington-Dulles)Segment 2: ARNK: Arrival Unknown or Surface Transportation from IAD to PHLSegment 3: 15-NOV: PHL/SFO (Philadelphia to San Francisco)