Treasure Class
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From Diablo Wiki
Treasure Classes, or TCs, were how the item drops were organized in Diablo II. It's not known if TCs will return in Diablo III, but even if they do all the item types and levels will surely be modified, so even if the same term is used there won't be any actual continuity of item types or levels.
The entire Diablo 2 item generation process is fully explained in the Item Generation Tutorial.
