Or rather, what is the criteria to be a Scion lucky enough to be whisked away to worlds unknown.

(SPOILER WARNING)

Basically, at the end of 4.4 and 4.5 part 1, a good portion of the main Scions have been put to sleep, Thancred, Y'shtola, Urianger, Alphinaud, and Alisaie. But what about other main Scions like Lyse or Krile or us, though we know why not us.

At first glance it looked like those who have trained in Sharlayan. Lyse, for all her involvement, wasn't trained like Yda. But then we see that Krile is still up and about. Someone made the suggestion that only those who have interacted with the Warriors of Darkness have been taken, which lines up, but why specifically them?

You will notice that with Alisaie and Alphinaud added to the others, the 5 NPCs that were shown to represent the Trust System are now all asleep, that can't be a coincidence.