The Ubuntu SDK appears to be only for creating Ubuntu Apps. What exactly is an "Ubuntu App"? The website is very detailed, but I can't see any description of what is meant by an Ubuntu App.
I understand that these apps can run on either the desktop or an Ubuntu phone, but which desktop? Is it specifically Unity, or will these apps be usable on Xubuntu, Kubuntu, Ubuntu MATE, or even Linux Mint?
Or does this depend on whether or not I use any Unity specific hooks?
Edit: the referenced website has been updated since I asked this, so the question is not really relevant any more.
I think that what an "Ubuntu app" is is really written in the explanation, i.e. a program that can work on Ubuntu (both Desktop and Touch).
With that SDK you can create QML and/or HTML5 applications.
This means that they can work on all the desktops that support these formats. I personally do not know a list of desktops with such capabilities, apart from the default Ubuntu interface, Unity.
The SDK is built thinking of Ubuntu (it is Ubuntu SDK, not Kubuntu, Xubuntu, Ubuntu Gnome, Ubuntu Mate, ecc) and gives easy access to the services provided by Ubuntu (such as Content Hub, as explained in the Platform section).
I guess that it can be easily adapted to write programs for other desktops that take advantages of QML and HTML5.
An "App" is a short name for "Application".
All it means is its a application that was built for Ubuntu, from a linux based SDK.