Back doors appear only in Kingdom Hearts coded and re:coded. They are mistakes in any worlds programming creating a scar like door to appear. Data Sora has a indecator for these where lights appear on the left and right of the screen and flash the closer to a back door you get. The lights will turn red when Data Sora has loctated the door. The back doors allow Data Sora to enter the core programming of the world to eliminate the 'bugs' inside.