3. Domains and System Space
Following are excerpts from the Unofficial DVD specifications Guide 2.2 ebook.

...Video and audio data are recorded as data structures on a DVD-compliant disc. These data structures are components of one of the four data groups called domains:

First-play (FP)
Video Manager (VMG)
Video Title Set (VTS)
Video Title Set Menu (VTSM)

3.1. Graphical display of DVD domains
...The following illustration visualizes the concepts of DVD domains. Each domain is displayed as a column in the following picture. From a hierarchy perspective, the data structure that is a level higher than titles or language units is the VMG (Video Manager) as shown in the Video Manager domain, or the VTS (Video Title Set) as shown in the Video Title Set domain. In other words, VTS is the top-level data structure in the title domain, while VMG is the top-level data structure in the VMG domain.

A group of titles and language units collectively makes up a VTS. A group of VTSes collectively makes up the title domain and the VTS menu domain. The DVD specifications permit up to 99 VTSes per DVD disc. Each VTS can have a maximum of 99 titles and the number of language units is limited by the currently supported language codes. Typical commercial movie DVDs have three or four language units to display different menus or subtitles for each language. The VTS menu domain is optional and may not be present on a DVD disc.

...The First-Play PGC and a PGC in the VMG can link to any title (in the title domain), any predefined menu (in the VTS menu domain), and any PGC in the VMG (in the VMG domain). A PGC in a VTS can link to...

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