Someone asked for this at the nexusforums and I figured it was something that could be useful later.
These show up at numerous occasions at the different tools. It applies to oblivion as well. This is what they mean:

ACHR= Character Reference
ACRE= Creature Reference
ACTI = Activator
ADDN = Addon Node
ALCH = Ingestible (Alchemy)
ALOC = Media Location Controller
AMEF = Ammo Effect
AMMO = Ammunition
ANIO = Animated Object
ARMA = Armor Addon
ARMO = Armor
ASPC = Acoustic Space
AVIF = Actor Value
BOOK = Book
BPTD = Body Part Data
CAMS = Camera Shot
CCRD = Caravan Card
CDCK = Caravan Deck
CELL = Cell/Area
CHAL = Challenge
CHIP = Chip
CLAS = Class
CLMT = Climate
CMNY = Caravan Money
CONT = Container
CREA = Creature
CSNO = Casino
CSTY = Combat Style
DEBR = Debris
DOOR = Door/Teleports or removes barrier
ECZN = Encounter Zone
EFSH = Effect Shader
ENCH = Enchantment
EXPL = Explosion
EYES = Eyes
FACT = Faction
FURN = Furniture
FLST = Form List
GLOB = Global
GRAS = Grass
GMST = Game Setting
HAIR = Hair
HDPT = Head Part
IDLM = Idle Marker
IAMD = Imagespace Modifier
IMGS = Imagespace
IMOD = Item Mod
IPCT = Impact Data
IPDS = Impact Data Set
KEYM = Key
LGTM = ImageSpace (Template)
LIGH = Light
LSCR = Loading Screen
LTEx = Land Texture
LVLC = Leveled Creature
LVLI = Leveled Item
LVLN = Leveled Character
MAPM = Mapmarker I guess yep
MESG = Message
MGEF = Base Effect
MICN = Menu Icon
MISC = Misc item
MSET = Media Set
MSTT = Music Type
MUSC = Music
NAVI = Navigation
NAVM = NavMesh
NOTE = Notes
NPC_ = NPC/Non Playable Characters
Pack = Package
PERK = Perk
PROJ = Projectile
PGRE = ?? Fiendmineref is used in the .esm
PWAT = Placeable Water
QUST = Quest
RACE = Race
RADS = Radiation Level
RCCT = Recipe (Collection?)
RCPE = Recipe
REFR = Reference
REGN = Region
REPU = Reputation
RGDL = Ragdoll
SCOL = Static Collection
SLPD = Sleep Duration
SCPT = Script
SOUN = Sound
SPEL = Actor Effect (Spell)
STAT = Static
TACT = Talking Activator
TERM = Terminal
TREE = Tree
TES4 = Remnant of TES4:Oblivion, just a header.
TXST = Texture Set
VTYP = VoiceType
WATR = Water Type
WEAP = Weapon
WRLD = World
WTHR = Weather