DD_DEFNS is a special lump recognised only by Doomsday. This lump can be used to include DED and reference_guide data within your wad to make distribution easier.
After all DED files (physical and virtual) have been processed, the engine will check through all the loaded WAD files for lumps named DD_DEFNS. All the lumps with that name are processed just as if they were DED files, i.e. they should contain a DED file in plain text format. The DD_DEFNS lumps are applied in the order in which they have been loaded.
The lump itself is a pure text format that can be edited in any text editor such as Notepad. However a lump editor is then required to extract or insert this data into a WAD.
Some editors capable of inserting DED definition files into a DD_DEFNS lump include: