Windows 8.1 is an operating system, a *specific version* of Windows. It[SQ]s not "for Windows" in the sense that it[SQ]s an add-on or an upgrade *to* a pre-existing version of Windows. It *was* a version of Windows, released as an update to Windows 8.

Think of it like this : Windows 8 was released, and then Windows 8.1 came along as a significant update with bug fixes, performance improvements, and new features. You didn[SQ]t install 8.1 *on top* of a different Windows, you either upgraded Windows 8 to 8.1 or installed 8.1 fresh. Windows 8.1 was a complete operating system in its own right. It[SQ]s no longer supported by Microsoft, meaning it doesn[SQ]t receive security updates.