Thing

In Doom, things are used to represent players, monsters, pick-ups, and projectiles. Inside the game, these are known as actors. There are three ways to get an actor into your map.

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.

Spawn numbers are the numbers used by specials such as Thing Spawn to spawn actors in the game once the user has started playing the map. Although DoomEd numbers and spawn numbers both describe actors, they are not interchangeable&mdash;you must not use a spawn number on a map thing, and you must not use a DoomEd number for instance as a Thing_Spawn parameter.

ZDoom editing
Actor Names are the actual names each actor has internally (to test these you may want to try out the summon console command). These are used with the functions SpawnSpot and Spawn which allow you to spawn any actor available with Thing_Spawn and also all other actors (though some actors aren't meant to be spawned) such as cameras, scenery or whatever you might require.

If you create a new actor with the DECORATE lump, you can specify both the spawn number (SpawnNum), and the DoomEd number (DoomEdNum). The name you give the new DECORATE item is also its actor name, so you can also spawn it using that.

THINGS structure
  Doom Format things are 10 bytes in size, while Hexen/ZDoom format things are 20 bytes in size.