Anandtech and other sites have information about Windows 10’s Compact OS feature.
Image from TechNet blog:
Excerpt from MSDN:
Windows 10 includes tools to help you use less drive space. You can now compress the files for the entire operating system, including your preloaded desktop applications. Compact OS lets you run the operating system from compressed files (similar to WIMBoot in Windows 8.1 Update 1), and single-instancing helps you run your pre-loaded Windows desktop applications in compressed files. The new processes helps maintain a small footprint over time by using individual files, rather than combining them in a WIM file. Compact OS installs the operating system files as compressed files. Compact OS is supported on both UEFI-based and BIOS-based devices. Unlike WIMBoot, because the files are no longer combined into a single WIM file, Windows update can replace or remove individual files as needed to help maintain the drive footprint size over time.
More information:
http://www.anandtech.com/show/9676/windows-10-feature-focus-compactos
http://www.windowscentral.com/how-reduce-windows-10-footprint
https://msdn.microsoft.com/en-us/library/windows/hardware/dn940129%28v=vs.85%29.aspx
http://blogs.technet.com/b/mniehaus/archive/2015/09/16/windows-10-reducing-the-disk-footprint.aspx
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