"Supports up to Windows 8.1" means it will work with every windows currently in use: windows 7 and 8/8.1.
Asrock can't say it supports Windows 10 yet, because Microsoft hasn't start the certification process.
So basically it will run any Windows 8/8.1 version, they're all the same, using the same kernel and drivers, it's just some features are disabled in the "lesser" versions.