Windows is no longer an operating system for only PCs, laptops and tablets. Core OS is the future of Windows.
Windows Core OS (or Wcos for short) is a new, stripped-down version of Windows that can be adapted to run on a wide variety of devices with minimal work compared to the old way of doing things. Oddly, there hasn’t yet been an official announcement about it from Microsoft - even now that the company has released a Windows 10X emulator for app developers.
The likely reason for this decision is that no-one really needs to know about WCOS. It’s an internal name for a software component that is only one part of any branded Windows product. Important? Yes. Interesting to the end user? Not so much.