B777 Captain reports routinely receiving only a partial random route clearance through Reykjavik airspace from Shanwick Control via data link and wonders why this is.

Narrative

Though this report is specific to this flight; the same narrative applies to a number of other flights over the last few months. Here is the datalink clearance we received from Shanwick: CLRD TO [DESTINATION] VIA ATSIX RANDOM ROUTE 64N020W 68N030W 70N040W FM ATSIX/1118 MNTN F360 M083 ATC/LEVEL CHANGE ENTRY POINT CHANGE END OF MESSAGE. The problem with this clearance is that it doesn't go all the way to Edmonton airspace. In fact; it ends part way through Reykjavik airspace. Here is another example; CLRD TO [destination] VIA BALIX RANDOM ROUTE 65N020W 68N030W 70N040W FM BALIX/1449 MNTN F360 M084 END OF MESSAGE. Same issue; the clearance ends part way through Reykjavik airspace. More importantly; there is nothing on the datalink clearance to indicate that this is a partial clearance; or that further clearance must be requested or received from Reykjavik Control or Iceland Radio. It would seem a very easy task for Shanwick to add this bit of free text to clearances like these: REQUEST FURTHER CLEARANCE FROM XXXX. I've seen many clearances like this recently; and have personal knowledge that a call to Iceland Radio or Reykjavik Control will get the remainder of the clearance. However; I recently flew with a First Officer who was new to the Atlantic Random Routes and was diligently studying the FOM on how to get the clearance. It became apparent that this 'partial clearance' is not addressed in the FOM and that there was clear confusion on the First Officer's part that we did not in fact have a full clearance via datalink from Shanwick. It took nearly an hour from our initial contact with Iceland Radio; asking for further clearance; to the time we finally received it from Reykjavik Control. I think the true burden lies with Shanwick to pass this tidbit along; however perhaps the FOM should address it too.