Windows 10 Lean was revealed on Twitter by Windows enthusiast Lucan, who noted the heavily cut-down OS has no wallpaper and is missing apps like Registry Editor and Microsoft Management Console, as well as drivers for CD and DVD drives. Due to this trimming, the x64 install of Windows 10 Lean is about 2GB smaller than Windows 10 Pro.
Microsoft also refers to Windows 10 Lean as Windows 10 CloudE, which has a few quirks, including Edge not showing download prompts, meaning it's impossible to download files, and for some reason Office can't be installed.
It's not clear what Microsoft's plans are for Windows 10 Lean/CloudE and how it fits in with Windows 10 S, which at one point was also known as Windows 10 Cloud.
Another intriguing inclusion in this Redstone 5 preview is the appearance of phone-calling APIs that would support standard phone functionality such as dialing, video calling, and blocking withheld numbers.
As noted by Ars Technica, Windows currently has limited support for mobile modems to enable data connections, but actual calling required Windows Phone or Windows 10 Mobile.
The new phone-calling APIs could support speculation about Microsoft's long-rumored plans around Andromeda, a foldable Windows 10 mobile device that could serve as Microsoft's return to the mobile market.