Reward Flight Saver - redemptions with a flat fee that is somtimes lower than the taxes/charges associated with a cash ticket (and would otherwise apply for the reward ticket too). RFS can be found on BA flights but since you're including an AY segment it would price the entire ticket at the regular rate (and not the BA leg with RFS, the AY leg with regular charges). Because of this it may work out cheaper to book them separately - but sometimes the flat RFS rate is actually more than the regular taxes/fees so it can be better not to use RFS! You just need to check which is better for your specific flights (plus the change/cancel fees that were referenced above).

https://www.britishairways.com/en-es...reward-flights