One fairly common issue people have is that older addons they install do not load when they log into the game. This is usually due to the fact that World of Warcraft tags older addons as out of date and aren't automatically loaded unless you tell the game to. This typically happens after a patch, as the game client tends to mark addons as out of date at that point.

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Often the name modification (or "mod" for short) is used interchangeably with AddOn. More specifically, a mod is a third-party AddOn that alters or enhances some aspect of the game's interface. Mods can perform a wide variety of tasks, including adding new buttons to the screen, altering the built-in player and party frames. In extreme cases, mods completely supplant the standard game's interface, usually offering more advanced functionality.