Actually, Windows 10 - so if I understand it correctly this is about universal apps and as such would mean that if you port your app succesfully it would immediately run on all phones/desktops/tablets/whatever running Windows 10? If so it seems like this could be a major thing given the amount of complaints I read about the lack of apps for windows phones, i.e. in the long run if this works out then it's goodbye Android/iOS-only apps and welcome Android/iOS/Windows apps?

Though oddly enough, the beta version only works for x86/64 architectures. So not Windows Phones.

I don't know how odd that is. Windows 10 Mobile isn't released yet, and Windows 10 has been released. Nobody is going to DEVELOP these on Windows Mobile devices, so it still needs to work on Windows 10 for dev and testing. So developers can start working on using this now, and a later update will add phone support, so they can deploy there as well.

The Candy Crush app is claimed to be a ported iOS app, already available on Windows Phone 8. And I kinda thought the whole Universal Apps thing was architecture-independent.