Mentioned somewhere else: Windows NT has subsystems. E.g. posix, os2, win32, ... What they do is adding a subsystem to windows. The module they code is the subs

Mentioned somewhere else: Windows NT has subsystems. E.g. posix, os2, win32, ... What they do is adding a subsystem to windows. The module they code is the subsystem not the distribution. Nowadays it is a bit blurry die to the full kernel and this windowing subsystem.