Using a generic key can be helpful if you wanted to install or upgrade to a specific Windows 10 edition for evaluation or testing on a PC or virtual machine, or just don't have a digital license or your genuine product key currently available and you will activate later when able.
The Windows 8.1 installer (when you install Windows 8.1 from DVD/USB) will detect the product key from UEFI firmware (BIOS) and activate Windows 8.1 automatically.
Can I reinstall the Free Windows 10 on my computer after upgrading? If you have upgraded to Windows 10 with the free upgrade offer on an activated Windows 7 or Windows 8.1 PC, you will be able to do a clean install of Windows 10 on the same PC anytime in the future and activate Windows 10 with your Windows 7 or Windows 8.1 product key.
This tutorial will show you how to find your Windows 8 product key from within Windows 8, a Windows 8 retail box, or online order confirmation email.
Product Key - Find for Windows 8 - Windows 8 Help Forums
The product key you enter during installation (Windows Setup) is what determines which edition gets installed. If you select both for the Architecture, you will need to use at least a 10 GB USB flash drive if creating Windows 10 USB installation media with the ISO.
Now I can not even attempt to customize Windows 8.1 preview, because it is not activated..... I am quite frustrated to say the least, and even attempted to install (clean) back to Windows 8 Pro with my purchased DVD....when I entered the same product key that was successful (only 3-4 days) ago....the installation process stalled!