m.room.join_rules

Controls who can join the room. This is best to configure in room settings.

knocking

Requires room version 7

{
    "join_rule": "knock"
}

membership of another room

Requires room version 8 9. Offers only Spaces in room settings

{
    "allow": [
        {
            "room_id": "!oHIvaRnuEEIDavQVRJ:matrix.org",
            "type": "m.room_membership"
        }
    ],
    "join_rule": "restricted"
}

membership of one of multiple rooms

{
    "allow": [
        {
            "room_id": "!psumPMeAfzgAeQpXMG:feneas.org",
            "type": "m.room_membership"
        },
        {
            "room_id": "!ddFNqIRUacwdicInJz:feneas.org",
            "type": "m.room_membership"
        }
    ],
    "join_rule": "restricted"
}

It can be presumed that multiple spaces is simply copying that more.