The DDF file is not part of the ASEG-GDF2 standard. It is specially created by INTREPID, to assist you importing this data back into INTREPID at a later date. If you have completed Importing data to INTREPID (G03), you will recall that using the INTREPID Import tool requires you to create a DDF (Data definition) file. This is the very same file that the Export tool has created. This makes life easier for INTREPID users!
See https://www.intrepid-geophysics.com/ig/uploads/manuals/documentation_pdf_intrepid/gtexport.pdf:
Details here: https://www.intrepid-geophysics.com/ig/uploads/manuals/documentation_pdf_intrepid/ddffrmat.pdf#M9.96903.h1.Chapter.4.A1.The.DDF.format
So for now, we ignore them.