What happens if for SpriteA I go
-- Pick farthest from SpriteB.X, SpriteB.Y
will it just pick the farthest SpriteA from the first instance of SpriteB? or does it choose a random SpriteB when I do that event?
Random, you must pick SpriteB also.
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It won't refer to a random one, it'll be the first created object. That means it consistently refers to one object instance, but it may as well be any of them. Read this article for more information, especially the expression rule.