Bookmarks are an option that allows the player to create a temporary save file in the middle of a chapter. Upon creating a bookmark, the player is forced to return to the game's title screen. The bookmark save can then be loaded at any time to allow the player to resume the chapter from where they left off. However, the bookmark will be deleted upon loading it, and it will also be deleted if the player loads a normal save file instead.
Bookmarks are primarily useful for if the player needs to take a break or otherwise needs to stop playing for whatever reason in the middle of a chapter, since they can use a bookmark to resume from where they left off, rather than restarting the chapter or leaving the game running.
Depending on the game and the selected difficulty level, the "Battle Save" feature may be available instead of bookmarks. Battle Saves do not force the player to return to the title screen, and they also are not deleted upon loading.