Originally Posted by
number_6
Forgot about the US being country of origin (I never buy US/Canada origin OWE tickets, in part because of this rule!).
Gardyloo is quite right, once returning to the US the additional 5 segments are limited to being in the US/Canada and do not allow travel to Mexico, etc. SJU is still part of the US, though.
The circumvention for the OP is to return FCO-LHR-YVR, which would allow the remaining 5 segments to be used anywhere in North America (as described in my original post; having not returned to the country of origin, the quoted rule doesn't apply). YVR is probably more convenient than routing through SEA, in any case! Certainly a nicer flight, and BA has always had one of their better services to YVR (nicer than to LAX, oddly enough).
OP should be aware that though the LHR-YVR flight is arguably nicer, its still a BA TATL and therefore does not earn AA miles.
Which brings me to another question: can different segments of a xONEx ticket credit miles to different OW partners? i.e., can the LHR-YVR TATL segment credit to AsiaMiles or BA Exec Club even if the rest of the ticket credit AAdvantage?