3.11. Video Title Set Menu VTSM domain
Following are excerpts from the Unofficial DVD specifications Guide 2.2 ebook.

...The Video Title Set Menu (VTSM) domain is where the menu PGCs and language units of all video title sets (VTS) on a DVD disc reside. Each language unit in the VTSM domain may contain the predefined menus (root, angle, subpicture, chapter, and audio) and other standard PGCs to support the display of submenu pages. The VTSM domain is not required to be present on a DVD-compliant disc if there is no menu. In this case, the associated VTS_nn_0.VOB file (where nn is from 1 to 99) can be omitted or specified as a 0-byte file.

PGCs within a VTS menu domain cannot link directly to another title or menu PGC in a different VTS. How this is accomplished is explained in the Unofficial DVD specifications Guide 2.2 ...

3.12. Graphical display of language units in VMG and VTSM domain
...The following illustration shows how a PGC from the VTSM domain links to other PGCs in the Video Manager VMG domain or in a different VTS.

...In the above diagram, arrows marked with the numbers in magenta represent the playback flow directed by a virtual command...

...JumpSS (as in case 1) links a PGC in the VTSM domain (root menu in this case) to a PGC in the Video Manager VMG domain. JumpSS can be cross-domained as long as the domains are a part of the system space (hence the abbreviation SS, as in 3)...

3.13. Virtual commands in VTSM domain
The following virtual commands are available in the VTSM domain:

VM Command	Description
1. Nop	No Operation.
2. GoTo	Go to a specified command line.
3. Break	Exit the current command section.
4. Exit	Terminate the playback of a video DVD.
5. LinkPGCN	Link to a PGC in the same domain.
6. LinkPGN	Link to a program in the same PGC.
7. LinkCN	Link to a cell in the same PGC.
8. Rsm	Resume playback.
9. JumpVTS_TT	Jump to a video title in the same VTS.
10. JumpVTS_PTT	Jump to a PTT in a specified VTS.
11. JumpSS	Jump to a PGC in System Space.
12. SetSTN	Set Stream Number.
13. SetGPRM	Define contents of a GPRM register.
14. SetGPRMMD	Set GPRM register/counter mode.
15. SetHL_BTN	Set Highlight Menu Button.
16. SetGPRM, Compare & Link	Compute GPRM, compare to another GPRM and Link
17. Compare, SetGPRM & Link	Compare GPRM, compute another GPRM and Link
18. Compare, SetGPRM, Link	Compare GPRM, compute another GPRM, Link