Python API: additional_properties?

The Python API contains a field 'additional_properties' (e.g. in ShapeDataForCreate).
What is it good for?

I was trying to add a reference ID (I am trying to generate a Miro digram based on on an external model),
 

scr = ShapeCreateRequest(data=ShapeDataForCreate(content="Yeah", 
                         shape="rectangle"),
                         additional_properties={"ref": 17})

api.create_shape_item(board_id=board_id, shape_create_request=scr)

but I got an error message:

HTTP response body: {
  "type" : "error",
  "code" : "2.0703",
  "context" : {
    "fields" : [ {
      "field" : "ref",
      "message" : "Field [ref] is not supported"
    } ]
  },
  "message" : "Invalid parameters",
  "status" : 400
}

So my questions are: What is additional_properties' intended use?
Can I add some extra data to an item (so that I can later query that information?)