Originally Posted by MatthewLAX

Same fare class or lower of the preceiding flight.

So, for example:

LAX-LHR J
excuristionst perk: J/Y
return doesn't matter, must just end in same region as origin

but something like:

LAX-LHR Y
excurstionst perk: Y only
LHR-LAX J

Doesn't matter that return is in J. What matters is the preceding segment.