Solaris 2 is an "operating environment" that includes the
SunOS 5.x operating system and the OpenWindows 3.x window
SunOS 5.x is based on USL's SVR4.0. SVR4.0, in turn, was
developed jointly by AT&T and Sun while Sun was developing 4.1.0,
which is why things like RFS, STREAMS, shared memory, etc.,
are in SunOS 4.1.x, and why things like vnodes, NFS and XView
are in SVR4.0. (RFS, by the way, was dropped effective Solaris 2.3).