That depends on which iso file you use, for example is you use VMware-VCSA-all-6.5.0-9451637.iso then you goto VCSA 6.5 U2C.
Here you find a good overview of the release and build numbers.
Thanks! Thought I'd looked everywhere
That's the chronology and build number mapping....
So the take away for me is, the upgrade tool is bundled with the upgrade ISO, so of course the version you get upgrade/migrated to is the version from the ISO bundle.