Here is what the page “Valve Adds Achievement Sets to Steam, Separating Base Game and DLC Trophies” contains. Valve has rolled out a new Steam feature called achievement sets, giving developers the ability to organize achievements into distinct groups for a base game and its downloadable content. The update changes how players browse their unlocked achievements, making it easier to track progress across different parts of a game. Instead of one long list, achievements can now be separated by set, such as the main campaign or a specific expansion. This is particularly useful for games with large amounts of post-launch content, where achievements from multiple DLCs can clutter the view.
Valve says players will still unlock achievements normally by playing the game; the change only affects how they are displayed afterward. The feature is optional for developers, but it could become a standard for titles with extensive add-ons. This listing links straight to the original page, where the content is available in full.