Originally Posted by
remyjette
https://www.britishairways.com/en-gb...ving-your-seat
For oneworld Emerald, "
Free seat selection from the time of booking
for everyone in your booking and friends and family on other bookings, who are travelling with you in the same travel class and on the same flight."
Thank you, [MENTION=10099355]remyjette[/MENTION]! That clarifies my booking. [Update: I booked BA flights on AA's site with cash.]
Now to clarify my friend's booking. 75K booked BA flights on alaskaair.com using AS miles and chose "Main" as the class of service, not "Partner Premium".
Over on ba.com after making the booking, friend was able to select Premium Economy seats for all segments, and was not limited to Economy.
Is that a known feature of booking oneworld partner flights using RDM on alaskaair.com, or did my friend end up able to book Premium when policy would have limited seat selection to Economy?
And Yes, I am extraordinarily envious. 
75K friend chose Main, not Partner Premium, and was able to book Premium Economy seats for the BA flights.