It might be confusing because the term "subsystem" isn't something people are familiar with. On Windows NT (Windows' kernel) a subsystem is a set of APIs and an environment that software can run under. On Windows you typically use Win32, Win64, or WinRT but NT is not limited to those, it can support basically any software if the environment is written for it. In the past NT also supported OS/2 and it even had an entirely different *nix subsystem called Interix. WSL is basically the replacement for Interix because it was extremely obsolete and was barely used at all.