For those wondering why the naming and the confusion it is because there was a time that they were completely different products. Cubase goes WAAAAAY back to the early days of sequencers, originally being for the Atari ST in like 1989, later coming to Windows and MacOS. It, of course, progressed and got new features including become a full fledged DAW, developing the VST standard, etc.
Then in 2000 Steinberg introduced a new program: Nuendo, targeted at post production. It was at least partially new code, and I am inclined to think almost entirely. It looked different, worked, different, and cost a ton, like $3000. The two product lines were separate code bases.
At some point they realized that maintaining separate code bases was work that didn’t need to be done, and Cubase VST as it was called looked dated, so in 2002 they released a new Cubase called Cubase SX and reset the version number to 1 (was previously 5.1) using the Nuendo codebase. They weren’t the same product though, and Nuendo had a lot more features and cost a lot more.
Nuendo also tended to lag Cubase in features. Cubase would get new stuff and only later would Nuendo get it. Nuendo has more features, but not always the newest. Nuendo didn’t get major updates as often as Cubase, you’d get more point releases, which made some sense with its price tag. So you’d have Nuendo 7 contemporary with Cubase 9, that kind of thing.
That all changed fairly recently, they renumbered Nuendo to match Cubase with 10, and made it more of a strict superset. They also dropped the price to $1000. Still expensive, but not as way above as before.
With version 11 they seem to have a pretty direct version matchup, Cubase doesn’t seem to be doing the “.5” versions anymore which Nuendo never did, and the patch levels even match, like right now both are on 12.0.70. Nuendo still (at least as of 12) launches slightly after Cubase, so a new Cubase comes out with new features maybe a month or so before a new Nuendo, but they are essentially the same product.
So when you look at it now, it can be confusing as to why there are different products, but it is because of a very long and winding history.