Valve is introducing a major new feature for Steam achievements called "achievement sets" that lets developers list different groups of achievements for a base game and DLCs.
"In games that support sets, you still unlock achievements by playing the game as usual," Valve says. "But when you browse what you have unlocked afterward, Steam will break the long list of game achievements into the developer-created sets, making it easier to see what you have completed and to find other related achievements you can unlock next." Achievement sets have their own progress bars, and if you finish a set, you'll get a "set complete" trophy that's visible …