Hi,
The new site wizard provides you with the new site creation.
If you select to import the files, they are imported - for example the images used on your web site (e.g. when storing files in file system, or when using ASPX templates, web parts files, etc.) The files come from the
site export package. The new site wizard is almost the same as the
site import.
Importing new documents - again, it is the same as in importing a site, the new documents which are not present are imported to the system.
I would like to recommend you to go through the user
tutorial first, to get the meaning how the system works and what its design.
You may be right that the description is too general, but anyone can import any site export package where can be any type of files, so we are not able to specify which files - in general and logically the must must be the files that belong to the site somehow.
Best Regards,
Juraj Ondrus