This is a good question. I think there are a couple of things to consider here.
From the visual studio 2015 project templates above, I see that they have only given a generic universal app template and not split the templates into WinPhone/WinRT as they have for Windows 8, and what they had done in VS 2013. So this means that all apps developed for Win 10 (at least from a coding perspective) automatically compile for all devices. You cannot make just a phone app or just a tablet app anymore (as this has obviously become redundant).
Therefore, the only way MSFT will allow developers too chose which platform they want to target is likely through the app store....where they ask the developers to manually chose which device(s) they want to target.
Interestingly though, I went to their build conference and they were touting how the new Win 10 platform can allow code re-use with very little effort on the developers part and the XAML will itself change according to the design.
Therefore I would assume, that most developers would want too target ALL devices. Because if we believe what MSFT is saying..they dont have to write new code for different devices.
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However, I can also foresee instances where it makes sense to just have an app on a single device. For example, certain hardware restrictions will mean apps that use the gyroscope for example..will not be available on the tablet.
But I think such apps will be the exception and not the norm.
P.S Sorry for the long post. Has microsoft started accepting apps in the Windows 10 store?