Returns the record that relid points to. The record type comes from reltype and sets the response key. Records are written with reltype Subscriber, so the response is the contact, under contact. The body is an empty object {} when the contact no longer exists or relid is 0. Users outside the Admin group also get {} when the contact is only on lists their group may not access. This read does not check the linked activity entry. Returns 404 if no record has the id.