4.5. PGC in the Video Manager VMG
Following are excerpts from the Unofficial DVD specifications Guide 2.2 ebook.

...Most entry PGCs in the Video Manager (VMG) domain are program chains without any cells but contain only precommands. The precommands receive results from the general parameter registers or system registers and make conditional branching to titles or menus of other video title sets (VTS) on the DVD. These PGCs are commonly referred to as "route-through" PGCs since they interpret results from a VTS domain and branch to a different VTS domain. This is useful since the DVD specifications do not allow linking directly from one VTS to another. Other uses of PGCs in the Video Manager are to display short video clips without audio such as FBI warnings, region incompatibility display, or copyright screens, etc. The use of PGCs with long video playback in the VMG is not recommended since the maximum size of the VMG (VIDEO_TS.VOB) is limited to 1 GB.

4.6. Graphical display of PGCs in the VMG

Details on how PGCs in the VMG interact with other PGCs in different domains are explained in the Unofficial DVD specifications Guide 2.2 ...