foreign

In the (abridged) Feature object shown below

{
    "type": "Feature",
    "id": "f1",
    "geometry": {...},
    "properties": {...},
    "title": "Example Feature"
}

the name/value pair of "title": "Example Feature" is a foreign member. https://datatracker.ietf.org/doc/html/rfc7946#section-6