Is the iTunes authorization on the Windows 10 PC tied to the MAC address, Windows boot drive volume, or both? Will reinstalling Windows count as a new PC even though the PC still has the same MAC address on the Ethernet or Wi-Fi controller?
PC-OTHER, Windows 10, null
I believe it's tied to the MAC address.
Why is reinstalling Windows not listed in the list on Deauthorize your computer using iTunes - Apple Support under you should deauthorize your computer before you sell it or give it away, send it in for service, or make major upgrades?
As you point out, the article does say:
You should deauthorize your computer before you:
I would consider reinstalling Windows a major upgrade, but that's just me.
If you made no changes to the hardware, I'm surprised iTunes didn't still recognize the device.
Windows authorizations are tied to the boot volume. In a multi boot PC each instance of the OS needs authorizing separately. OS X manages to authenticate the machine. Authorization may survive a Windows upgrade, but I'd recommend deauthorizing beforehand and reauthorizing afterwards just in case. Reinstalling Windows will clear away any active authorization record so you should always deauthorize before reinstalling.
If you're running low on available authorizations, and are not able to use the deauthorize all command because you've used it in the last year then contact iTunes support and explain the problem. They should be able to reset things for you.
tt2
Deauthorize iTunes
