With the release of Windows 10 Insider build 17650, Microsoft added a new stripped down version called "Windows Lean" that uses approximately 2GB less space on x64 systems. It achieves this by not installing many of the standard apps and features that are usually installed in Windows 10.

Windows Version Selection Screen
Source: https://twitter.com/tfwboredom/status/987400226913779712

This new version was first spotted by Windows expert Lucan when the new Redstone 5 insider build was released.

As you can see from the screenshots posted by Lucan, the Windows 10 Lean version does not include many of the normally installed apps and features. Instead, placeholder buttons are displayed that indicate you can download and install an app or feature as necessary.

Missing Apps & Features
Source: https://twitter.com/tfwboredom/status/987400226913779712

When installed, this version provides a few clues in the Windows Registry as to what Microsoft may be planning with this version. According to the ProductName value, Windows Lean is part of the "Windows 10 S Insider Preview" branch and has a CompositionEditionID, or subid, called "CloudE".

Registry Information
Source: https://twitter.com/tfwboredom/status/987400226913779712

Lucan pointed out, though, that this version does not have the normal restrictions that are normally found in Windows S.

While this is an interesting development, like many of the new features discovered in Windows 10 Insider builds, there is currently no clear cut answer as to what Microsoft plans for Windows 10 Lean. At this point, we will just have to wait and see.

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