If you are using an official Windows 8.1 ISO downloaded through the Media Creation Tool, you will need a valid product key. For preinstalled Windows systems, this key is often embedded in the BIOS/UEFI and will be detected automatically. For retail copies, you must have your original product key available.
The remaining files are packed using extreme compression algorithms (like LZMA2 via Windows 8.1 Highly Compressed 600mb
To achieve this extreme reduction in size, custom developers strip out various elements, including: Windows Media Player and secondary multimedia codecs Multi-language support files (leaving usually only English) Pre-installed Windows Metro apps and games System recovery tools and help documentation Redundant device drivers for obsolete hardware Key Features and System Requirements System Requirements If you are using an official Windows 8