Achievements can have the following labels applied to them:
SecretThese achievements are considered secret and will be hidden until unlocked
DLCThese achievements require downloadable content either as a free title update or purchased DLC pack
OnlineThese achievements require some sort of online multiplayer, online game mode, or general online interaction
UnobtainableThese achievements have never been made available to unlock legitimately
DiscontinuedThese achievements can no longer be unlocked due to a server closure, in-game bug, or update
Main StorylineThese achievements are assured to unlock naturally during the initial playthrough of the main game mode or storyline
MissableThese achievements require additional steps or could easily be skipped outside the natural progression of the initial playthrough of the main game mode or storyline
CollectibleThese achievements require you to explore and obtain individual unique items in the game environment
LevelThese achievements require you to level up using in-game mechanics, including achievements that require you to reach a specific level, rank or experience threshold
LuckThese achievements require a definitive amount of luck to obtain, especially those that are only unlocked through RNG or random occurrence
CumulativeThese achievements require you to do certain actions a certain amount of times
DifficultyThese achievements require you to play on a certain difficulty
StackableThese achievements can be obtained at the same time or as part of, other achievements
ViralThese achievements are obtained by spreading them among other players
Time-LimitedThese achievements require some sort of individual action or collection of actions to occur on or within a certain time and/or date limit
We are so glad you asked! If you come across a game, achievement or DLC listing that you feel is out of place or incorrect, you may submit corrections using the Submit Game Information, Submit Achievement Information or Submit DLC Information links located on game, achievement and DLC profiles.
These links are typically located in the sidebar of each respective profile, however you may also find these links located in other areas for greater visibility. Alternatively, you may also let us know by creating a thread in the Issues & Bug Reports forum.