Licensed Microsoft Windows 8/8.1 systems:

A properly licensed Windows 8/8.1 system has a Genuine Microsoft Label (GML). Windows 8 systems do not have a Certificate of Authenticity (COA) label.
This includes Windows 8/8.1 Pro systems that have been downgraded to Windows 7 Professional.
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Microsoft Reimaging Rights (OEM Win8):

Customers who purchase OEM licensed versions of Windows may only restore the original factory preload using the Recovery Media provided by the OEM. Customers do not have any rights to create a custom image from the OEM preload and then deploy that image on OEM licensed systems. Customers can customize the preload on a particular system by adding or removing software, as they choose, but that the customized image may not be used on any other system (each system would have to be customized individually). Customers who wish to purchase OEM systems, with a custom image, may do so provided that they are purchasing 25 or more systems over a 12-month period. These systems would be custom models and not generally available in the market.
Download Reimaging License Brief