Vob Ids

[jaa52]

I really got lost or I am missing something simple. In the guide for "How to keep only the main movie of a DVD". It says use VOB Extra button. Now it brings up the audio choices and subpicture (english)
Next is the part I am missing, please explain.

It states:

Here I keep VOB IDS 2 and 3 which contain the main movie. How is that decided. I really would like to completely understand this key area.

John

[Doom9]

when in doubt it often helps reading the guide again. Step 2 has a link to this page: http://www.doom9.org/mpg/analyzeDVDcontent.htm#ex1