It’s in page 25 of the PDF, or if you want view the web version of the doc: https://www.ibm.com/support/knowledgecenter/en/SS63NW_9.5.0/com.ibm.bigfix.lifecycle.doc/Lifecycle/SWD_Users_Guide/c_edit_a_package_or_fixlet.html
Click Use Relative Path if you want to add the files into a specific folder of the package, and then enter the folder directory. For example, you must enter InstallPackage/Languages if you want to add another language file to your package with the following structure:
InstallPackage
- install.exe (file)
- Languages (folder)
- jpn.srt (file)
- eng.rt (file)
- Cache (folder)
Basically just a different way of organizing the files in your package.