ODF 1.2-1 5.4.1 "If a section is linked to another document"?

In Part 1 section 5.4.1, General, for <text:section> it is stated that

"Sections support two ways of linking to external content. If a section is linked to another document, the link can be through one of the following ... ."

But there appears to be no alternative to linking. According to the schema, there must be exactly one of <text:section-source> or <office:dde-source> element as the first element child of every <text:section>.

So what are the alternatives to "linked to another document"? If there are other sources, what are they? If there are <text:section> elements with no sources, how is that accomplished?