B777-200
Flight Crew Air Transport Pilot (ATP)
Communication Breakdown; Situational Awareness
ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural FAR
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Human Factors; Procedure
We received our oceanic clearance from Gander.  It was an amended clearance including a different coast out point.  We accepted the clearance and used the [LORAN] guide to check it.  We input the revision and checked it.  We then proceeded direct to the new coast out point.  About two minutes later Moncton center asked if we were going to the new coast out point.  When we responded affirmative center advised us that the oceanic clearance was just that and did not include a domestic clearance to proceed to the coast out point. At the time we were using Controller Pilot Datalink Communications (CPDLC) for both domestic center contact and the Oceanic clearance and we thought that we were doing what was expected.  There was no other deviation and no loss of separation with any other aircraft.This was a misunderstanding as to the action required by a clearance issued via CPDLC.  I think had we gotten the clearance via CPDLC and we were talking to Moncton on the VHF rather than monitoring Moncton on VHF as directed via CPDLC there would have been clear delineation between agencies but in this case we didn't catch the difference.This exact scenario is being given in this year's [training].  Highlighting this would have prevented this.  I don't remember asking for a clearance to the new coast out point during the [training] either.
A B777-200 Captain reported a track deviation on an international flight stemming from a misunderstanding with ATC.