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On a DVD, instructions for interaction between the user and the presentation engine is in a special Bitstream called the Navigation Stream. The navigation stream gives the player information about what to do when the viewer presses various buttons on the remote. The navigation stream is multiplexed along with the video, audio, and subpicture streams.
Every DVD player has two basic components, the navigation engine and the presentation engine. The user interface, as defined by the navigation stream, interacts with the navigation engine. The navigation engine then sends information to the presentation engine which determines what will content will be played.