The Journal is a core element of the Amnesia games, and serves as an item where all found documents are stored.
All Notes and Diaries found in Amnesia: The Dark Descent are stored in Daniel's journal where they can be reflected upon. Also in the journal are his Mementos, which are hints and objective reminders. Since Amnesia: Justine is just a small expansion for the game, the journal has the exact same model and serves the same purpose. Justine only has Notes to pick up, so no Diaries are present. Mementos aren't used either, since the puzzles are all intuitive enough to solve, with some of the Notes giving additional information.In Amnesia: A Machine for Pigs, Mandus' journal looks quite different, and now has two categories: Found Documents, which contains everything he picks up, and My Journal, which serves the same purpose as the Mementos.