I found
this on the Apple site, but it did not help me so I filled in the survey and suggested something to them.
"Again, you don't have to worry about figuring out which kind of update is best for you, just let Software Update preferences do the work." My question is what is SU does not work???? Then I do have to worry!
It seems that Software Update chooses the version you need based on what you have. My problem is that Software Update is not accessible behind a proxy, so I have to do a manual download - but which one??