Seems that we do, and seems I was wrong. So /F BC tells Matrix to use the cheapest fare that both includes the leg(s) in the fare and uses the prime RBD specified on at least one leg of the fare, not to actually book it in said booking class (ie downgrades allowed, etc).

Disagreement was mostly that it doesn't "set a fare" because Matrix is free to use any fare that will satisfy those things. /F CC.AAA+BBB.FFFFFF sets fares as in tells Matrix use fare FFFFFF on cx CC for AAA-BBB.

By the way, any chance to get O:[flightnumber] implemented? I think that would be a most useful improvement, especially now that we're looking at more JVs, ie BA/IB/JJ/LA for EU-LATAM and BA/IB/QR for EU-wherever QR flies.

eta:
And ofc thank your for all the info too, sir!