[QUOTE=C Elegans]Set items are like normal items in the sense that the less good will drop more often, and the really good ones will drop very rarely. How often you get a certain set item also depends on the items drop class. Drop class is quite complicated, but for instance you will get Isenheart's case (set breast plate) and Sigon's helmet, gloves and armour very, very often while you may never get Tal's armour, IK helmet or Natalya's claw.[/QUOTE]
Treasure Classes are fairly simple actually.
The game contains Treasure Classes (TC's) starting from TC3 all the way up to TC87 (they go up in 3's, i.e. TC3, TC6, TC9 etc.), there are also TC's for runes, rings, amulets etc.
Every time you kill a monster the game will make some decisions, firstly whether or not the monster will drop an item or not (the probability of a no drop can be manipulated by the number of players in a game),
Secondly if the game decides it will drop an item it'll pick a TC to do so. Every type of monster has a specific table of TC's that it can drop from, (superuniques and bosses have their own seperate drop tables), these tables also contain the probabilities of a monster dropping from that particular TC.
Thirdly the game will check the qualitly of the item dropped. That is the game will try for a unique, set, rare, magical ~ the odds of increasing the quality of an item can be increased by wearing more %magic find equipment.
To illustrate the point, Hell mephisto can drop up to TC78 (this TC includes items like scrouges, crusader bows, balrog skins etc) so you have a chance that meph will drop a stormlash, eaglehorn or arkine's valour. The problem is that meph's chance to drop from TC78 is very low.
Baal on the other hand can drop from TC87, (his chance of doing so is very low though) so Baal can drop sacred armours, hydra bows, colossus blades etc, so kill Baal enough times and you'll see a Tyrael's, Windy, and Grandfather drop.
There is a bit more to it than that (Qlvls, mlvls and area lvls) but in a nut shell that's TC's!
