After starting with Ubuntu, Microsoft has added a number of Linux distributions to its Windows Subsystem for Linux (WSL) Linux runtime environment. A Windows machine can simultaneously offer an Ubuntu, SUSE, Debian, and Kali “personality,” providing users with a choice of the different distributions’ preferences and package management.
But if your distribution isn’t yet available or if you want a Linux installation that’s customized just the way you like it, there’s now an answer: Microsoft has an open source tool for building your own Linux package. The tool is aimed at two groups: distribution owners (so they can produce a bundle to ship through the Microsoft Store) and developers (so they can create custom distributions and sideload them onto their development systems).
It’s still not really Linux though, is it? It’s the GNU userland and various tools, the Linux kernel is not involved. It would be more fitting to call it GNU/NT or GNU/Windows (depending on how pedantic you want to get).
That’s not meant to take away from how useful a tool it could be; I suppose for some it’s easier and faster than dual-booting between Windows and a *nix of choice for dev work.