There are more TCs than just the TC3... TC87 that most people are familiar with. Almost all item drops go through the TC structure in some way. Generally, a TC is defined as a list of sub-TCs with a chance of dropping from each one.
Each monster has a base TC (special monsters like Baal or the Countess have their own one, but most drop from a more general group of TCs (eg Champions, Uniques, Casters, Wraiths, etc). The grouped ones may upgrade to a higher level one in the same group in NM and Hell.
These TCs then break down into sub TCs like Equipment, "Good" items, "Junk" items, Potions, etc.
The Equip TCs further break down into the TC3-87s that are bandied about. Good TCs break down into Jewellery, gems, charms, runes...
The entire structure is defined in a file called TreasureClassEx.txt. It isn't too hard to understand what is going on just by reading it, each TC is listed as a row, and each row lists the smaller TCs (or at the lowest level, individual items) and the relative chance of each one.
An example. Suppose you kill a Fallen in Hell Cold Plains. That Fallen's natural TC is "Act 1 (H) H2H A" (from monstats.txt), meaning it is found in the early part of Act 1 Hell and is a Hand to Hand monster. Act 1 (H) H2H A is a level 63 TC in TC group 7 (which is all the H2Hs). Because the Fallen is actually mlvl 68 (area level of Cold Plains), the TC will upgrade, to Act 1 (H) H2H C. Not a big jump, but still worth having.
Act 1 (H) H2H C is defined in TreasureClassEx.txt as having no-drop 100, gld 21, Act 1 (H) Equip B 16, Act 1 (H) Junk 21, Act 1 (H) Good 2. This means that on a random roll of 1 to 100+21+16+21+2 = 160, there is 100 chances of nothing dropping, 21 chances of gold, 16 chances of an equipment item from Act 1 (H) Equip B, 21 chances of a junk item and 2 chances of an item from Act 1 (H) Good.
Act 1 (H) Equip B is listed higher up as having chances of dropping from TC57 up to TC66, so the best equipment item that Fallen can drop is from TC66. It also has a (very high) chance to drop down to a lower Equip TC with lower TC items, and this can continue all the way down to TC3.
Act 1 (H) Good has a chance of Jewelry C, each Gem type up to Flawless, and Runes 13. Jewelry C contains all rings, amulets, jewels, and the three charm types. Runes 13 contains runes 25 and 26 (Gul and Vex) plus a (very high) chance of dropping down to Runes 12. So a Fallen could drop a Vex, although it is much more likely to drop down, probably all the way to El.
There is other info in the TCEx file, mostly affecting chances of a dropped item being magic/rare/etc, which is also a property of the TC.
Just to give you an idea of how the probabilities work out, note that initially you have a 2/160 chance of the drop from the Fallen going to the Good TC. The Good TC has a 14/130 chance of going to Runes 13. Runes 13 has an unappealing 2/8825 chance of dropping a Vex. So altogether your Fallen has a 56/183560000, or about 1 in 3.3 million, chance of dropping a Vex.
Each monster has a base TC (special monsters like Baal or the Countess have their own one, but most drop from a more general group of TCs (eg Champions, Uniques, Casters, Wraiths, etc). The grouped ones may upgrade to a higher level one in the same group in NM and Hell.
These TCs then break down into sub TCs like Equipment, "Good" items, "Junk" items, Potions, etc.
The Equip TCs further break down into the TC3-87s that are bandied about. Good TCs break down into Jewellery, gems, charms, runes...
The entire structure is defined in a file called TreasureClassEx.txt. It isn't too hard to understand what is going on just by reading it, each TC is listed as a row, and each row lists the smaller TCs (or at the lowest level, individual items) and the relative chance of each one.
An example. Suppose you kill a Fallen in Hell Cold Plains. That Fallen's natural TC is "Act 1 (H) H2H A" (from monstats.txt), meaning it is found in the early part of Act 1 Hell and is a Hand to Hand monster. Act 1 (H) H2H A is a level 63 TC in TC group 7 (which is all the H2Hs). Because the Fallen is actually mlvl 68 (area level of Cold Plains), the TC will upgrade, to Act 1 (H) H2H C. Not a big jump, but still worth having.
Act 1 (H) H2H C is defined in TreasureClassEx.txt as having no-drop 100, gld 21, Act 1 (H) Equip B 16, Act 1 (H) Junk 21, Act 1 (H) Good 2. This means that on a random roll of 1 to 100+21+16+21+2 = 160, there is 100 chances of nothing dropping, 21 chances of gold, 16 chances of an equipment item from Act 1 (H) Equip B, 21 chances of a junk item and 2 chances of an item from Act 1 (H) Good.
Act 1 (H) Equip B is listed higher up as having chances of dropping from TC57 up to TC66, so the best equipment item that Fallen can drop is from TC66. It also has a (very high) chance to drop down to a lower Equip TC with lower TC items, and this can continue all the way down to TC3.
Act 1 (H) Good has a chance of Jewelry C, each Gem type up to Flawless, and Runes 13. Jewelry C contains all rings, amulets, jewels, and the three charm types. Runes 13 contains runes 25 and 26 (Gul and Vex) plus a (very high) chance of dropping down to Runes 12. So a Fallen could drop a Vex, although it is much more likely to drop down, probably all the way to El.
There is other info in the TCEx file, mostly affecting chances of a dropped item being magic/rare/etc, which is also a property of the TC.
Just to give you an idea of how the probabilities work out, note that initially you have a 2/160 chance of the drop from the Fallen going to the Good TC. The Good TC has a 14/130 chance of going to Runes 13. Runes 13 has an unappealing 2/8825 chance of dropping a Vex. So altogether your Fallen has a 56/183560000, or about 1 in 3.3 million, chance of dropping a Vex.
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