1 SliTaz can't and doesn't want to provide non free software. Many people want to use software such as Opera, Skype ☎ or flash-plugin. SliTaz provides some get-* scripts to let you install them. The DVD creation tool will launch this script, repackage the software and store it in the package directory.
2 Some software such as OpenOffice or LibreOffice are so huge that they don't compile on a SliTaz build bot. The DVD creation tool will use get-* scripts (see above) to store these packages in the package directory. The scripts download pre-built binaries for another Linux distribution.
3 Original source files. Install SliTaz wok to get the SliTaz patches.