Redmond provides full details on Windows 10 activation
With Windows 10, Microsoft introduced a new activation system called digital entitlement that allows users to upgrade from an older operating system, such as Windows 7 and 8.1, to the new one without the need for a product key.
Once the upgrade is complete, the system automatically activates (assuming you were running a genuine version of Windows 7 or 8.1), with Microsoft then allowing you to clean-install without asking for a product key anymore.
Windows 10 is re-activated every time you reinstall thanks to this digital entitlement system that attaches a product key to your system (manual re-activation is needed when you make significant changes to your PC configuration, such as replacing the motherboard).
