Outlines Import Box
When you perform import of outlines, you should see a pop-up dialog box of the following example. In this dialog box, you can select to import outlines from the listing of source files, or select to from other PDF file on your hard drive. To follow, you can select further to import different part entries from one outline you selected.
Note: The "listing of source files" lists those pdf files of which page(s) have been included in or imported to Pages pane of PDFPO.

Displaying all bookmarks of a source file when you select it on the Source File(s).
This place is for listing source files which pages have been imported to the Pages pane. Double click a file will display its outline on the left side.
This place is for imputing outlines from other PDF documents on your drive. Using Browse, you can bring up an Open dialog box to find your intended PDF documents.
Illustrating the reference meaning of the icons on the Source Outlines.