2.7. Program Chain (PGC)
Following are excerpts from the Unofficial DVD specifications Guide 2.2 ebook.

...As previously explained, a cell is a basic unit of playback of video and audio data. Each cell is uniquely identified by its starting and ending sector address on the disc. A Program Chain (PGC) defines the order in which cells are played back and how they are played back depending upon the current settings of the DVD player.

One or more PGCs can be linked or chained to gether to form a video title (hence the term Program Chain). When there are more than one PGCs per title, one and only one of the PGCs is marked as a title-entry PGC. When a JumpTT is executed, the title-entry PGC is branched to (explanations of JumpTT is covered in subsequent sections). In simple movies, where one title has only one PGC, the cells recorded on the disc are played back in the same order as the cells in the PGC...

...Each PGC may have up to 128 pre-commands, which is executed before playing back the first cell in the PGC, and up to 128 post-commands, which is executed immediately after playing back the last cell in the PGC. The PGC may have cell commands, which can be executed each time a cell is played.

The total number of commands (including pre, post, and cell commands) cannot exceed 128 commands per PGC. Through these commands and user interactions, one PGC can branch into other PGCs, multiple PGCs can branch into a single PGC, etc., providing a rich environment for interactive playback...

...A PGC keeps a count of the number of programs and cells present in the PGC. If the number of programs/cells is 0, the PGC does not have any playback data and may contain only the pre- or post-commands. It is legal to have an empty PGC (having no cells and no command section) which does nothing. A PGC can be defined in this manner to direct and control the playback of video and audio data.

A PGC contains Prohibited User Operations to disable certain user interactions during the playback of cells in the PGC. Each PGC has a color palette of 16 indexed colors in component color format (YCrCb). Four indexed colors are used for subtitle, selection, action, and display color. Since a PGC is the most important element of the DVD specifications, additional sections are dedicated to discuss the detailed functions and structure of a PGC.