4.2. First-Play PGC
Following are excerpts from the Unofficial DVD specifications Guide 2.2 ebook.

...The First-Play PGC is the first thing a DVD player looks for when a DVD disc is loaded into the DVD player. In general, the First-Play PGC is an empty PGC with no cells and contain precommands to link to the first video clip that resides in the Video Manager or to a title or a root menu of a different Video Title Set.

The First-Play PGC is physically located in the VIDEO_TS.IFO which is a part of the Video Manager (VMG) but due to its importance and unique functions, it is considered as part of a separate domain, the First-Play domain.

4.3. Common virtual commands found in the First-Play PGC
...In the above illustration, when power is pressed to turn on the DVD player, if there is a DVD video disc, its first-play PGC is played back. The first-play PGC of most commercial DVD titles has a precommand section which contains one of the following types of virtual commands: JumpSS or JumpTT.

Details of virtual commands used in the above illustration are explained in the Unofficial DVD specifications Guide 2.2 ...