ExtraData is a text mode lump type in Eternity, designed as a predecessor to UDMF. It allows extended definitions for map things, linedefs or sectors when the map itself is designed in the classic Doom format. An ExtraData lump (custom-named by the user) is attached to the map via the EMAPINFO lump. The extended ExtraData object definitions written in the lump can be accessed in the map by placing specific thing types or linedef specials, making the corresponding records in ExtraData override the specific objects.
Features that require ExtraData's extension include, among others, thing TIDs, parameterized linedef specials and various sector special effects (custom damage, terrain types).
The upcoming UDMF map format for Eternity will no longer need an attached ExtraData, because UDMF fulfills the same purpose of arbitrary developer extensibility. ExtraData is only relevant for maps edited in fixed-size binary formats such as classic Doom or classic Hexen.