Kingdom Hearts HD II.8: Final Chapter Prologue follows HD 1.5 ReMIX & HD 2.5 ReMIX on Playstation 3. Those contained two full games and an HD movie based on a DS game. This compilation follows the same formula but is slightly different in execution.
On November 26th, 2016, HD II.8 was made available for pre-order on the Playstation Store. This marks the first Kingdom Hearts game to be available digitally.
Dream Drop Distance was originally released for the Nintendo 3DS, this is a full playable edition of the game. It takes place after Kingdom Hearts II & Re: Coded and leads into Kingdom Hearts III. It appears to be a straight port of the game with no new content, though touch screen mechanics will be altered to play on a single screen. This version will mark the first time a Kingdom Hearts game will run at 60 fps.
Kingdom Hearts χ [chi] started out available in Japan as a browser game before being remade as Unchained χ [chi] for mobile, which was released worldwide. Back Cover is an HD movie, about 80 minutes in length, that will trend over some territory as the previous versions but will expand on the previous versions with new content as well. While the playable versions tell the story from the player's perspective, Back Cover tells the story from the perspective of the major characters The Foretellers. It takes place the farthest back in the timeline the series has gone to date.
Birth by Sleep is a prequel that takes place 10 years before Kingdom Hearts. It was originally released for the PSP and was also included in HD 2.5 ReMIX. The Final Mix version contains a playable secret ending starring Aqua fighting the Heartless. This is a brand new playable episode (series director Tetsuya Nomura stated it's about the length of a single Kingdom Hearts world), that takes place some time after the original secret ending but still before Kingdom Hearts. It will combine gameplay systems from Birth by Sleep and Kingdom Hearts III.
Unlike all other new content in the previous Kingdom Hearts HD compilations, Back Cover & A Fragmentary Passage are being made in Unreal Engine 4, the same engine that Kingdom Hearts III is using.