How to determine which side of the division region UI should be placed

When iPhone Duo is partially folded, you need to move some UI to the left or right (or top or bottom depending) half of the screen to avoid the division region, but how do you determine which side is appropriate? Sheets move to the left automatically but why? Is there anything that should move to the right? Can you share any examples? Is there API to get the system recommendation for a specific type of UI or something?

Looking over documentation, the only thing I’ve uncovered for moving custom views is “get an array of UIView.ReservedRegion instances from reservedRegions(kind:options:). Inspect the frames of the reserved regions, and adjust your views accordingly.”

How to determine which side of the division region UI should be placed
 
 
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