PGCDemux: extract one PGC VOB vs. multiple VOB ID's per PGC

[LigH]

Based on a VideoHelp thread, I got a bit confused about the features of PGCDemux and the meaning of structure units on a DVD Video.

I assumed that PGCDemux is able to extract one PGC to a continuous "PGC VOB" file. To achieve that, you will have to check two boxes (one of them depends on the other):

[√] Create a PGC VOB
+ [√] One file per VID (if this is not checked, you will get VOB files split at each GiByte).

Now, the DVD in the linked thread apparently has several VOB ID's in a PGC. This makes PGCDemux split the PGC VOB at their break. So I tried to read in the WWW what a VOB ID is, at all ... with only little results: It is used to group cells in a PGC. Okay. But why? No reasons explained under which circumstances one may need more than one VOB ID per PGC.

I wonder: Is it related to the Layer Break?

So I hope you can tell me reasons why some DVD's have more than one VOB ID per PGC, and if PGCDemux has good reasons to split the extraction here, or if you can make it ignore that and create a VOB output across VOB ID's for the whole PGC. If not ... then it may require different software to do so. Or concatenate the parts after extraction.