3.8. Title domain
Following are excerpts from the Unofficial DVD specifications Guide 2.2 ebook.

...The Video Title Set VTS domain is also known loosely as the title domain. This is where the PGCs of all VTS titles reside. Each video title may contain one or more PGCs in a particular video title set. The DVD specifications limit up to 99 titles per each VTS, up to 99 Video Title Sets per disc, and up to 99 titles per DVD disc.

Even though the video title sets reside in the the title domain, PGCs within a VTS cannot link directly to another title in a different VTS with the command JumpTT. To accomplish this direction of playback, the virtual command CallSS is used. How this is accomplished is explained in the Unofficial DVD specifications Guide 2.2 ...

3.9. Graphical display of title domain
...The following illustration shows how a title PGC from one VTS links to another PGC in a different VTS...

3.10. Virtual commands in VTS title domain
...The following virtual commands are available in the title domain. The complete list is presented on page 26 of the Unofficial DVD specifications Guide 2.2 ...