cinder/api-ref/source/v3/samples/get-default-type-response.json

{
    "default_type": {
        "project_id": "6685584b-1eac-4da6-b5c3-555430cf68ff",
        "volume_type_id": "40ec6e5e-c9bd-4170-8740-c1cd42d7eabb"
    }
}