These passive abilities will usually take the form “[Game Object] can’t be chosen by [Category] spells and abilities.” Example: Ruin Runner’s passive ability reads “I can’t be chosen by enemy spells and abilities.”
§ 757.1 Core Rules PDF (officieel)
§ 757 Some Game Effects declare that a Game Object can’t be chosen by a certain set of spells and abilities. Those effects designate the Game Object as Untargetable.