Thing

Things represent players, monsters, pick-ups, and projectiles. Inside the game, these are known as actors, or mobjs. They also represent obstacles, certain decorations, player start positions and teleport landing sites.

While some mobjs, such as projectiles and special effects, can only be created during play, most things can be placed in a map from a map editor through an associated editor number. When the map is loaded, an actor that corresponds to that number will be spawned at the location of that map thing. See thing types for a listing of all things that have an associated editor number.

THINGS lump
Level thing data is stored in the lump. Each entry is 10 bytes long. This format is used by both Doom and Heretic.

Boom flags
Boom adds the flags:

MBF flag
MBF adds the flag:

Hexen Format
Hexen extended upon the things lump. Hexen things may contain an action special which is executed upon death or pickup of the thing (for monsters and items, respectively). Hexen format also introduced true z-clipping, allowing placement of monsters on the z-axis. Finally, Hexen monsters are also able to have a tag similar to sectors. This is called the thing ID or TID.

Note that Hexen things are 20 bytes in size; 10 bytes larger than things in Doom.

Strife Format
Strife did not alter the thing structure like Hexen did, however it did make use of the unused flag bits.

Doom 64 format
The Doom 64 THING format is similar to the Doom format except with the inclusion of a z position and an ID. Each entry is 14 bytes long.