Unclear sentence in 2.1.1 of ODF 1.0

Note that <office:document> may contain all supported top-level
	elements. None of the four subdocument root elements contain the 
	complete data, but four combined do.

What does "but four combined do" mean?  Four subdocument root elements 
collectively contain the complete data?


-- 
MURATA Makoto (FAMILY Given) <>