2.8. Graphical display of Program Chains
Following are excerpts from the Unofficial DVD specifications Guide 2.2 ebook.

...This document uses the convention and graphical symbol to display a program chain (PGC) as shown in the above illustration, representing its data structure as defined by the DVD specifications. A PGC can have up to 128 pre-commands and post-commands. A PGC does not require to have pre- or post-commands. The pointers to the pre- and post-command section in the PGC are set to 0 if there are no commands in these sections. The label "Pre" is shown on the left of the PGC icon to indicate the pre-command section. The label "Post" is shown on the right side of the PGC icon to indicate the post-command section. If there are commands in the section, the background of the label is displayed in color other than white. From a hierarchy perspective, PGC is a level higher than the programs. In the above illustration, the PGC labeled "PGC 1" contains three chapters spanning four programs. Program 1, 2, 3, and 4 has 2, 1, 1, and 3 cells, respectively. All cells except the first are seamless...

...PGC is one of the most important data structure of the DVD technology. Explanations of different types of PGC are illustrated in the Unofficial DVD specifications Guide 2.2 ...