Solaris 2 is an "operating environment" that includes the SunOS 5.x operating system and the OpenWindows 3.x window environment.
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).