What is the purpose of “additionalItems” property on arrays in rjsf?

I’m studying rjsf documentation and I’m confused about the additionalItems section of array docs.

Here is the example code from the docs:

const Form = JSONSchemaForm.default;

const schema = {
  type: "array",
  items: {
    type: "string"
  },
  additionalItems: {
    type: "boolean"
  }
};

ReactDOM.render((
  <Form schema={schema} />
), document.getElementById("app"));

and here is the official codepen

The rendered form seems to behave exactly the same if I remove the additionalItems, so what is the purpose? I guess it has one, since it’s explicitly brought up in the docs, but I can’t figure it out 🙂