Anyone who downloaded and installed Windows 10 Redstone 5 Build 17650 for Skip Ahead may have noticed references to the previously unheard of Windows 10 Lean. Microsoft has made no announcement about this, so WTF is Windows Lean?

As you might guess from the name, this is a version of Windows 10 where the focus is on reduced size. With an install footprint around 2GB smaller than Windows 10 Pro, Windows 10 Lean is a stripped back version of the operating system with unnecessary extras cut out.

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So what's missing from Windows 10 Lean? Microsoft has reduced its size by trimming out drivers, wallpapers, and a number of built in programs and tools. While the command line is present, tools such as the Registry Editor are missing.

Apart from the missing components, Windows 10 Lean is not restricted in any way, and there's little to stop users from adding the missing tools manually if they want.

The existence of the new version of Windows was first spotted by Twitter user Lucan, who shared images of his findings and said "Welcome to Windows 10 Lean/CloudE/S (once again?) This new edition started shipping with this week's Skip Ahead build (17650). It seems to be heavily cut down, an x64 clean install is roughly 2 GB smaller than Pro. Its edition ID is 0xB7 which was missing from SDK headers":

Welcome to Windows 10 Lean/CloudE/S (once again?)

This new edition started shipping with this week's Skip Ahead build (17650)

It seems to be heavily cut down, an x64 clean install is roughly 2 GB smaller than Pro

Its edition ID is 0xB7 which was missing from SDK headers pic.twitter.com/2Sn3SVXeZB — Lucan (@tfwboredom) April 20, 2018

Another interesting snippet of information is that once installed, Windows 10 Lean runs in Window 10 S mode, but its identifier is Windows 10 CloudE. It's not clear whether this could be indicative of Windows 10 Lean's possible future direction and Microsoft's target market, but it certainly could be. The operating system could be destined for very low-powered machines, it could be aimed at the cloud, or there may be enterprise applications that are yet to be revealed.

With Microsoft not giving anything away at this stage, all we can do is speculate.