Treasure Class
This is the system that Diablo uses to internally predetermine which monster can drop what item. While the basics of Treasure Class makes sense, it can be difficult to completely grasp the finer details of it. One thing about this sytem is that it never alters the way it works unless Blizzard were to change it. One could say that the game itself already knows which items can drop where and by whom, the rest is left down to what the player actually does and the [RNG] when it comes to deciding what will drop specifically.
The Basics
The easiest way for the game to grant gear progression is by limiting what can drop where. Similar to the idea that you will not face the most difficult enemies when you take the first steps out of the Rogue Encampment, you will not be seeing the best weapons until late in the game. This simple concept makes the game more difficult as you play, but also offers the gear to offset the difficulty later on. These two factors will create the main progression of the game, letting players feel they are fighting strong demons while wielding the rarest artifacts in the Sanctuary realm.
Treasure Class, also known as TC, is another attribute within the game mechanics that will never be refered on to players. It exists solely to help the game organize its inner functions. Every single item in the game will have a TC assigned to it, along with a TC number that can be compared to tiers. The TC itself marks what kind of item can be dropped, the TC number will group a more specific list of items that belong in that group. For a quick explanation, lets assume you just killed a monster and the game is now computing on what to drop (Info may not be accurate for the sake of explanation):
- The monster has an assigned Armor TC as well as a Weapon TC. Armor gets chosen.
- The monster can drop either TC3 or TC6.
- TC3 is composed of Quilted Armor and Leather Armor
- TC6 is composed of Hard Leather Armor and Trellised Armor.
- The RNG chooses TC6 Hard Leather Armor to drop.
- The game will now run its course on deciding what quality or magical status this item will have, if any. The fact that any one version of Hard Leather Armor will drop is undisputable at this point however.
While the above info is not accurate, it explains how it is that the game can pick something to drop from a monster. An analysis of the above six bulletpoints, we can conclude a whole list of things. Every single monster will have a preassigned TC. These will differ depending on where the monster's spawn point was located at, the monster's race, its boss status and the difficulty level of the game. Not all monsters will have all TCs at their disposal when it comes to dropping items. Some monster races may not ever be able to drop weapons and armor, strictly because they have no Weapon and Armor TC assigned to them. Next, we notice that a monster can have multiple Armor TCs assigned to it, where the highest valued TC resembles the better, more progressed gear. The highest TC value usually scales very close to what part of the game you are in, so that items within that TC are amongst the best you can find for that difficulty range. On the downside, the top TC will get picked less often than the tiers slightly below the max. This keeps players from finding the best gear all the time, and when something exceptional does drop, it won't get replaced immediately most of the time. Something else that catches our eye is that multiple items can occur in a TC. This is because, as mentioned earlier, a TC is a grouping of items that have a similar base level. Lastly, we find that the game will decide upon the quality or magical status last. This is important for people who plan to use their character to Magic Find. This is because MF will not simply make Harlequin Crest Shako drop. To find said item, first you must be lucky enough to have a Shako drop at all. Only then will MF affect the drop. This is ultimatley why it is good to understand at least a little bit about Treasure Classes before you start looking for a specific item.