Defining a Default Page for Detailed Navigation 
To set up a default page for detailed navigation:

Result
If a portal user clicks on a folder in the second level of the top-level navigation for his role and it does not have a page, this default page is automatically started in the content area.
The following also occurs:
The system makes sure that there is only one default page for detailed navigation. If a page is already marked and you mark another one, the marking for the first page is automatically reset.

Importing pages into the Enterprise Portal can result in more than one default page.