I have used code to test whether a child record and a parent record have to be in the same zone. They do.
Now... Does a parent record have to be in the same CKDatabase as a child record?
Does a parent record in CloudKit have to be in the same CKDatabase as a child record?
I don't think it's possible to save a record in one database and another record in a different database at the same time, so the parent record and the child record I mention in my post would have be saved in different lines of code. I think. I don't think it is possible to save two records in two different databases using CKModifyRecordsOperation. I don't know if there is a way at all.