To accommodate JSON message body structure requirements, each operation defined in a JSON (RESTful) service interface supports only one top-level body-type input and one top-level body-type output interface field; therefore, all interface fields that appear in the body of a JSON message must be children of a top-level body-type field.
In a single operation, a top-level body-type interface field represents the outermost structure of a JSON message body, and is known as a root structure. A root structure in a JSON message body is unnamed by design; however, it must still be represented in the operation. To accommodate this requirement, the Interface Mapper provides a special type of interface field called an anonymous root field. An anonymous root field is always the parent field of a root structure.
An anonymous root field appears in the Interface Fields pane with a Yes in the Anonymous Root column.