So anything that makes your aerial, after the initial "Guarded" hit, but before Clark actually does his grab animation.
Conversely, this also means that oppoments can use this to their advantage. For example, you can use B SAB as an anti-air to step jump-ins, but also, people Mai and Hwa, who can cancel into secondary air actions, can get the drop on you.
If a K's approaches you with his "SHADAAAAA!" Minute Spike, you can absorb the Minute and Narrow Spikes, and throw him, if he does the second hit instantly, but if he staggers it, or stays in the air, foregoing the Narrow Spikes, you'll be left open.
I do think that's the supreme beauty of B SAB, though. It trains people to attack in ways they wouldn't, and makes them react in risky ways that Clark can take advantage of. Standing D, Gattling attack, Standing C, and just hopping after and grappling all beat their backdashes, but EX SAB, GP SAB, and D SAB all make staying still or attacking on wakeup so dangerous...