I understand your confusion and do not have the answer. But I think the PlusPAK, when added to your existing 2019 UltraPAK, will get you everything in the 2020 UltraPAK. I think the reason they don't list MIDI Supertracks, MIDI Styles, etc. in the PlusPAK listing is because those things are either 1) NOT updated for 2020 or 2) if updated they are included inside the free bonus pak or the $49 pak. So, you get the bonuspak with PlusPAK and to get the other stuff you need to buy the additional 49PAK in any case.

So, what I always do, even though it is $50 more expensive, is I go ahead and order the UltraPAK upgrade because they ship everything on a hard drive and my installation is simpler and less error prone because I don't need to add it to my existing installation. And if I ever need to reinstall I just grab that hard drive.

WARNING: The UltraPAK PLUS includes songs and lessons but these have not been updated for 2020 (or in any recent release AFAIK) so if you bought them previously you already have them!!!