Okay, it's all a bit fuzzy. The spec assumes 30040 is always an article and each linked element is a section of the article. But we also allowed for 30040 to be a book and each linked element is then a chapter. And we allowed for 30040 to be a collection and each linked element is then an article, in it's own right. I made a 30040 that was an audiobook list and each 30041 contained an audiobook episode.

I actually prefer 30040 index and 30041 section, as we don't presume to know the content of the events, we're just describing what the "do": one contains a list of events, the other contains a portion of the content.