SunOS was BSD-ish originally, then around 1994 they shifted to a more SVR4 variant and rebranded to Solaris. I think Solaris 2.x was SVR4 and Solaris 1.x was BSD retronaming of what was SunOS named earlier.

It's been a while so I may not have the numbers quite right.

Anyway, there was some overlap in nomenclature, so a version bump of the SunOS naming (and Solaris numbering) implied a shift from BSD-ish to SVR4-ish, as I recall.

(And I worked for Sun for a while after that, and used Suns a ton around the shift)

"SunOS is the kernel, Solaris is the distribution as a whole" was how I was always told it went. Solaris 1.x used a SunOS 4.x kernel, Solaris 2.x and upwards used SunOS 5.x. Version synchronicity was achieved by jumping from Solaris 2.6/SunOS 5.6 to Solaris 7.x/SunOS 5.7 in 1998 when UltraSPARC support was introduced. Of course Solaris 11.x/SunOS 5.11 was the last version.