4.7. PGC in a VTS
Following are excerpts from the Unofficial DVD specifications Guide 2.2 ebook.

...PGCs (program chains) in a video title set (VTS) must conform to the defined parameters set for the VTS, such as the video format (NTSC or PAL), the type of encoding used (MPEG-2 for video, AC-3 for audio), encoded bit rate, etc. PGCs contain the required playback and navigation information to play video objects and audio streams in a particular order designed by the DVD author.

A video title in a VTS may contain one or more PGCs. These PGCs are numbered so that the DVD player knows that they belong to the same video title. The video title set keeps track of the order and physical locations of the PGCs within a VTS title. This makes VTS-domain instructions simple and efficient to access a relative PGC within a VTS title...

4.8. Graphical display of PGCs in a VTS
Details on the following illustration regarding possible virtual commands in the title domain (CallSS, JumpVTS_TT, JumpVTS_PTT, and LinkPGCN) are explained in the Unofficial DVD specifications Guide 2.2 ...