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: