The answer to your question is no, there is no published rules on the upgrade priority. There are FT's that claim they know the proper "coding" something like (2UPA or UP2A) needed for a SWU, but I was told by four Delta employees (when I kept escalating insisting I was right) when a full Y got the last seat on a flight I was using a SWU that that does not put you at the top of the list and that full Y fares trump a DM on a SWU.

No one has ever been able to produce an official document from Delta with what is the true priority.