repos/zypper - way of working

Choosing correct repos seems to be a big issue in this forum.

When to run zypper up and/or zypper dup together with some choosen repos depends upon which state of the upcoming openSUSE I which to try out. Correct?

Are there some defined way of working describing what to suspect from the different repos?
In general there are the openSUSE-11.2-Oss and openSUSE-11.2-Non-Oss repos, that I guess holds the M* and RC* releases and finally the stable 11.2.
And then we have the fact-oss and fact-non-oss repos. If I get it right these should contain all of the above repos and sometimes more. Is it enough for me, trying to run on latest fact, to only use these repos?
The packman repo have been pointing out the fact-packman repo, but are now rebuild (empty for a start?). Are the other repos build first and pointed to then to be more accessible? Or are there downtimes to be considered?

zypper up/dup
As the repos is updated instead of creating new repos between RC1 and RC2 for example, what is the difference between zypper up and zypper dup? What does zypper dup add? My guess for the purpose of zypper dup was to make zypper aware of the complete change of repos.