We'll be dropping it per tom's recommendation, but... there exist JSON specs that have fields that are allowed to be some type of thing, or a list of that same type. The practical example is the HAL spec where, for a resource, each link relation in the _links property may have one link or a list of many. In either case the structure of a link should follow one definition. That scenario is your basic practical example of a type-choice and this field implements it.