Thanks for your reply. The terms & conditions on the eMCO itself don't say anything about partial redemptions. And I did find the same site you cited above, but it seems to be talking about refund requests after having already completed travel, but it's unclear whether this also applies to redeeming online during booking. That's why I said I couldn't find anything "definitive". If you or anyone else here can confirm that those T&Cs apply in all cases, it'll be great.