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Doomsday Engine Definitions, or DED, is a content definition language used by Doomsday and Risen3D that can be loaded as plain text files or from within a WAD.
When embedded in a WAD, the definitions must be placed in a lump named DD_DEFNS, with the exception of XG, which can read from lumps named either DD_DEFNS or DDXGDATA
All of Doomsday's modding and visual features, from creating new mobj types and XG line/sector types to model and dynamic light definitions, are defined in DED files.