Version - First Doom version that thing appears in:
S - Shareware
R - Registered
2 - Doom II
Radius - Radius, in map units (note that the map "footprint" of a Thing is square, not round, so this is the distance from the center to the midpoint of an edge)
Sprite - First four letters of the name of all sprite graphics
Sequence - For animated sprites, default pattern through which the animation progresses. "+" indicates that certain gameplay developments (such as a monster being shot) may interrupt this pattern with additional graphics
Class:
A - Artifact item. Counts toward ITEMS percentage at the end of a level
P - Pickup. Player can pick the thing up by walking over it
O - Obstacle. Players and monsters must walk around
^ - Hangs from ceiling, or floats if a monster (aside from Commander Keen, who does not move; dead monsters, excepting Lost Souls, fall to the floor when killed)
Only one of these may be picked up per level, although any additional maps still count against the player's item tally. In multiplayer it is possible to pick another after dying and respawning.
FIRE is used when a spawn cube turns into a monster.
TFOG is used while a teleporter is operational.
A certain number of additional objects (such as spawn cubes, rockets in flight, and Spider Mastermind corpses) may not be placed in a level at its outset, but are generated only during gameplay.
Trivia
There were several THINGS that were removed before the full public release. These Include:
A "Zapper"
An Office Chair
A Broken Lamp
Two lamps
Three Empty Barrel types (with Empty Nukeage Barrel, Brown Barrel and Grey Barrel)