Ah I see. So it is in the same boat as dthrow fair?
think of it more like a paper scissors rock kinda thing.
you downthrow
your opponent chooses not to DI? You get a Fair
Your opponent chooses to DI in? You get an Uair
Your opponent chooses to DI out? You get a Bair
^ not quite always like that, but in general its something like that.
Dthrow to RAR Bair is pretty much safe for fox to throw out without having to worry about getting hit, so in the early stages of a set you can just go for it and see what your opponent does, typically you'll see these sorts of setups start working more and more when watching these top players as a set progresses and they start figuring out their opponents habits and start picking the right options. Anti being a top player himself will also be aware of this and will be trying hard to not be predictable so sometimes you wont see the options work at all.
And I think, but I'm not 100% sure, that on some characters at specific percents a down throw can true combo to a RAR Bair, but those situations wouldn't be super common, and sometimes a player might just be throwing it out just in case it does true combo, or in case the opponent is lazy and doesn't bother to DI properly.