UWP is ambiguously defined and the acronym's usage is on the way out. An evolution of UAP and Windows Store apps, it boiled down to a whitelist of APIs allowed on multiple devices (the U in Universal) and encompassed Win32, WinRT and COM APIs. UWP itself isn't an entity with APIs, is not a technology, and the original concept is dead thanks to new platform leadership at Microsoft. (Still in play on Hololens, Surface Hub, and other sandboxed devices for now...)

That said, "UWP" is confusingly also used to refer to Windows apps that tap into WinRT APIs, participate in the modern app lifecycle, are modeled and have identity, or generally tap into newer technologies such as AppContainer. You'll see this, for example, perpetuated in Visual Studio in its project templating. It's a BIG OL' MESS right now; I'm working with/pushing Microsoft to get this communicated clearly and mopped up but old habits die hard.

Well, it is quite clear actually.

https://docs.microsoft.com/en-us/windows/apps/desktop/choose...

These docs are outdated and incorrect, sadly. For example, nearly everything under the UWP heading can be done via traditional Windows apps. Current leadership seems to get it, "write an app, use any APIs you need" (rough paraphrasing). Just hasn't fully set internally and externally so these warts exists.

On a technical level, there are still some apps that can get CoreWindows, respond to PLM events, receive callbacks from contracts involving CoreWindow UI like Share target, as well as go into certain UI modes involving shell participation like modern fullscreen and CompactOverlay, and some that can't (without stapling them together with one that can via Desktop Bridge) ... aren't there? Maybe that will change in the future but it hasn't yet.

And then there are some OS editions where some apps can't run or can only run in a container environment.

Waiting to see that message at BUILD 2020 or the upcoming Ignite.

Until then, sorry to say but I follow up on what is available to us.