Thing

Things represent players, monsters, pick-ups, and projectiles. Inside the game, these are known as actors. They also represent obstacles, certain decorations, player start positions and teleport landing sites. See Thing types for a listing of all things.

DoomEd numbers are the numbers used in the editor to represent an actor. When the map is loaded, an actor that corresponds to that number will be spawned at the location of that map thing. Although many actors have DoomEd numbers, not all of them do. For instance, a blue key card has a DoomEd number, but a rocket flying through the air does not.

THINGS lump
Level thing data is stored in the THINGS lump. This format is used by both Doom and Heretic.

Boom flags
Boom adds the flags:

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.