Lean is, in fact, the name of the Microsoft distribution and will be a trimmed down version of Windows 10, It lacks many Windows 10 features and has a 2GB smaller installation size.

Build 17650, the Redstone 5 preview released last week, has a new lightweight version of Windows 10 called Windows Lean. Windows 10 Lean was revealed on Twitter by Windows enthusiast Lucan, who noted the heavily cut-down OS has no desktop wallpaper (disabled by default), the Microsoft Management Console and registry editor are missing, drivers for CD and DVD drives can't be installed, Microsoft Edge doesn't allow downloads, and Microsoft Office is missing as well.

A number of existing 16GB of storage devices struggle to update to the latest Windows 10 updates. Consumers typically load on apps, media, and files that prevent Windows Update from having the necessary free space to upgrade to feature updates of the operating system. We’re expecting to hear more about the Windows 10 Lean edition and Microsoft’s plans for the future of Windows 10 at the company’s Build developer conference in early May.

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.

Microsoft to release a Lean Version of Windows 10