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The Windows Subsystem for Linux is now open source.
(blogs.windows.com)
This is a most excellent place for technology news and articles.
I think it is because Windows has many subsystems, it's just that you don't hear about most of them aside from WSL.
So it is referring to the particular Windows Subsystem (of which there are many) that can run or emulate Linux.
Wait, Windows still has POSIX subsystem or is it only listed for documentation reasons (it was there at least in old NT days)?