How are achievements structured in online multiplayer games?

The only information available in the verified source concerns the definition and general characteristics of massively multiplayer online role‑playing games (MMORPGs). It describes how players assume character roles, the persistence of the game world, and global revenue figures. No details are given about in‑game reward systems, achievement mechanics, or how such features are organized or implemented. Therefore, based on the evidence at hand, we cannot provide a description of achievement structures. Any further explanation would be speculative and not supported by the cited material.