Seville died in the end of 2003, Deville died in the end of 2005.
Note that the "model year" runs from about July to June, so a new 2020 XYZ4-V could have been built from July 2019 to June 2020.
STS stood for Seville Touring Sedan
SLS stood for Seville Luxury Sedan
DTS stood for Deville Touring Sedan
DHS stood for Deville High (luxury) Sedan (some say it was Deville Highway Sedan)
The 2006 - 2011 "STS" and the post 2005 "DTS" are not acronyms, they are totally new models, and the letters do not stand for a model name - they're early GM alphabet soup that means nothing.
Some of the first 2003 CTS had polymeric bumper parts that were molded with "Catera Touring Sedan" part numbers, but that went away in a hurry and the "name" stayed "CTS".
Anything after 2006 or so in the Cadillac line is EUROLAC alphanumeric gibberish and carries no defining meaniing.