Currently modern-ui is described as:
Modern UI, officially named "Windows Store" apps, is how we refer to the new design language and style used in Windows 8 (and Windows Phones)
It's an obsolete term for Windows Store Apps and we have a tag for those: windows-store-app
A Windows Store app is a program distributed through Microsoft's Windows Store.
I think we should make modern-ui a synonym for windows-store-app.
Of course with this metro and metro-ui, both of which are current synonyms for modern-ui (and should be) will need to become synonyms for windows-store-app.