Well, in something like Shogun, there are MANY substories intertwined with the main story (a Portuguese pilot stranded in Japan who falls in love with an “off limits” woman and becomes embroiled in local conflicts while trying to get himself and his shipmates back home). Adding substories (which can include sub-plots, sub-characters, sub-themes, and even sub-genres) adds a richness and density to the work, but are not essential to relating the “message” (storyform) of the main story.