2.12. PGCs in a language unit.
Following are excerpts from the Unofficial DVD specifications Guide 2.2 ebook.

...A program chain (PGC) contains commands to direct playback. The DVD specifications impose restrictions on certain types of virtual machine commands which a PGC can have depending upon where the PGC resides. As previously explained, a group of PGCs can be chained together to create a title. The principal function of a video title is playing back the main movie features with minimum interaction from the viewer, so it makes sense to prohibit the execution of certain virtual commands when a title is being played back. For example, the command to set the parental management level is not allowed while a video title is being played back. This restriction reduces the possibility of command errors created by the authoring process and also lessen the hardware requirements to decode virtual commands by the DVD player. Virtual commands which support the highlighting and selection of menu buttons are available only for the PGCs which are a part of a defined structure called Language Unit...

...A PGC in a language unit usually contains commands to perform interactions with menus to highlight and to select certain items on the menu. The DVD specifications allows a PGC to be one of the following predefined menus when it is a part of a language unit: title menu, root menu, subpicture menu, angle menu, audio menu, and chapter menu. Without these menu attributes, a PGC in a language unit has all other functions and properties of a standard PGC...

...Explanations of the VMG and VTSM domain are covered in subsequent sections of this guide. The section PGC Category has more information on the usage of predefined menus in language units with a standard remote control. Please also read Language Codes for a complete list of the languages supported by the DVD specifications.