Throughout the POSIX specification, there's provision (1, 2, 3...) to allow implementations to treat a path starting with two / specially.

A POSIX application (an application written to the POSIX specification to be portable to all POSIX compliant systems) cannot assume that //foo/bar is the same as /foo/bar (though they can assume that ///foo/bar is the same as /foo/bar).

Now what are those POSIX systems (historical and still maintained) that treat //foo specially? I believe that POSIX provision was pushed by Microsoft for their Unix variant (XENIX) and possibly Windows POSIX layer.

It is used by Cygwin which also is a POSIX-like layer for Microsoft Windows. Are there any non-Microsoft Windows systems? OpenVMS?

On systems where //foo/bar is special, what is it used for? //host/path for network file systems access? Virtual file systems?

Do some applications running on Unix-likes —if not the system's API— treat //foo/bar paths specially (in contexts where they otherwise treat /foo/bar as the path on the filesystem)?