Occasionally while doing the Tempest Trials (I don't think its specific to that, but it's where I've been noticing it happening), I see a light blue tile when trying to move a unit. It happens with Sonya most, if not all, of the times I've seen it. For reference, this is what I'm referring to (the square directly above Sonya):
What does this tile mean? I don't think it represents a tile I can attack from, because it doesn't show up in most other cases where this would be applicable.