4. PGC categories
Following are excerpts from the Unofficial DVD specifications Guide 2.2 ebook.

...Each program chain (PGC) contains information and commands on how to play back a list of cells. However, PGCs are not created equal. Depending upon where it resides on the DVD disc, there are restrictions imposed upon its commands and requirements that the PGC has to respond to certain playback functions invoked through the user remote control. The following illustration shows different categories of PGCs. In general, a PGC falls into one of the following ten categories. Details of these categories are explained in the Unofficial DVD specifications Guide 2.2 ...

...In the above illustration, the icon of a user remote control key when placed next to a PGC icon indicates that the PGC is played back when the user presses the corresponding key on his remote control unit. The arrows in magenta indicate the flow of virtual commands at the precommand or postcommand section of a PGC. The numbers (1 through 6) in magenta indicate typical virtual commands used to direct the flow and branching of playback. The domains are highlighted to show the placement of a particular category of PGCs which are allowed by the DVD specifications...

4.1. Common virtual commands
JumpTT is used to direct playback to a specified title in a VTS. JumpTT is commonly issued from a PGC in the VMG domain or from the First-Play PGC.

LinkPGCN is used to link from one PGC to another in the same domain. The domain can be the VMG, VTS, or VTSM domain.

CallSS is used to direct playback from a PGC in the title domain to a predefined menu in the same VTS, or to a PGC in the VMG domain, or to the First-Play PGC.

JumpVTS_TT is used to direct playback to a title in the same VTS. This is the same as linking to the entry-PGC of the title.