Rule 355.10.d
It is programmatically selected based on its characteristics rather than chosen by the spell or ability’s controller. e.g., “Kill all units at a battlefield” targets a battlefield, but does not target any units. e.g., “Kill all units at battlefields” doesn’t target anything. e.g., “Kill a unit. Its controller draws 2” targets the unit, but not its controller. e.g., “Ready your legend” doesn’t target anything, because you can only have one legend. e.g., “Ready a friendly legend” targets a legend, because in a 2v2 game there are two friendly legends. e.g., “Recycle all cards in your trash” doesn’t target anything, because it affects all cards and you only have one trash.