There is no official policy but it works as intended. If i buy a AF light fare i can select an standard economy seat on any DL operated flight within that reservation. I have to call DL to get access to the "preferred" seats but that usually works fine. No access to E+ seats untill OLCI when these become available to everyone. Still for free with status. So this is how it should work. The "Hack" is the other way around: if i'm on a basic economy fare bought from DL i can select seats on AF/KL operated flights as these carriers don't have fares which prohibit advance seat selection.