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IMO, the usage of the VOBs depends mainly of the authoring program. I have noticed, for example, that on many commercial DVDs, the VOB ID changes for the layer break cell, but it's not at all mandatory. (The LB must be at the beginning of a new cell, but not necessarily at the beginning of a new VOB.)
Other than that, if a domain has several PGCs with different video content, it is of course mandatory to use a different VOB IDs for each clip. (It's why PgcDemux has its "One file per VID" option. It's handy to split a "play all PGC" to its different parts, such as episodes. But as you know, it will also split at most layer-break cells, even if you don't need that.)
IIRC, it is also mandatory to use different VOB IDs for the different angles of a multi-angle movie, but honestly, I don't remember exactly.
I don't think there are other cases where a VOB change is mandatory.
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