I came across a good explaination recently from Joel.
Snip….(from Joel’s blog – see his for more details)
Next, what prescan is looking for…
-
Customized
site templates – You need to know which site templates have been customized for
a particular site so you can verify the customizations again after the upgrade. Check
out the site
template upgrade kit for details on these. -
Sites
based on custom site definitions (if you don’t run the correct command for SPS 2003,
you’ll see all your areas)
-
Unghosted
pages – with URL so you know which sites to focus on (it won’t fail if you have these,
just report them.) -
Orphaned
objects Objects such as list items, lists, documents, Web sites, and site
collections that have been orphaned. Correct
these before upgrading.
-
Custom
Web Parts Report the existence of custom Web Parts to the appropriate
site administrator or developer before upgrading, to give the administrator or developer
time to investigate.
-
Sites
that are based on languages or that use controls that are not installed, note these
will result in failures if you try to ignore.