3.5. Video Manager (VMG) domain
Following are excerpts from the Unofficial DVD specifications Guide 2.2 ebook.

...The Video Manager (VMG) domain contains language units to hold the title menu and other predefined structures to have access to any elements of each VTS available on the DVD disc. The title menu is displayed when the "Title" button on the remote control is pressed. The title menu is used as a top-level menu through which the user can access root menus from other video title sets (VTS) on the disc, submenus in the VMG, and video titles available in video title sets. There is no requirement for the Video Manager domain to have a title menu. The VMG domain can be left empty with no PGC (the VIDEO_TS.VOB is not required and can be omitted or defined as a 0-byte file). If a PGC is present in the VMG, whether it is a title menu or a standard PGC, at least one language unit is required.

The following illustration is explained in the Unofficial DVD specifications Guide 2.2 ...