I don’t know how they are chosen, I think that’s the “matching” step though, so it’s likely that they are calculated somehow.
I’ve read before that taking images in a row would be better, but not how or why (as usual). I could imagine that it is mainly for speeding up calculations since RC might check first if the next image fits to the current one and if not needs to search all others to find a match. But again, that’s mostly speculation…