I don't think every last piece does need to be made
accessible separately. You are right, not everything can be
written in a modular fashion. It requires a conscious
design on the part of the author. The author must know
what levels will define topics and provide context where
necessary. For that reason, it cannot be done automatically
by, say, a server. Many topics could have subsections
contained in them. In that particular example, the
document is large and deep enough so that the <S3> could
stand alone, and it might contain <S4> section within the
file. Shorter documents might only make sense chunked at
the <S2> level.
All I was asking for is author *control* over how the
hierarchy gets broken up and reassembled.