Nowadays, SunOS is the operating system
that forms a part of the Solaris operating environment
. A so-called Solaris system
is in fact running a SunOS kernel
# uname -a
SunOS sappa01z 5.7 Generic_106541-12 sun4u sparc SUNW,Ultra-Enterprise-10000
Note what the first word says. This domain is running Solaris 7.
Before the release of Solaris as a bundled operating environment, SunOS was a BSD style unix. With the release of Solaris came the switch to a SYSV style.