When topics are organized as subtopics of other topics, our tools generate a list of the subtopics at the end of the parent topic. If the DITA source of the subtopic begins with a <shortdesc> element, the contents of the <shortdesc> appear under the subtopic title in the parent topic. These contents also form the first paragraph of the subtopic. It does not seem to be possible to suppress the list of subtopics; they appear in the HTML output, but not in the PDF.
Generally, the documentation is not consistent in its use of the <shortdesc> element.
I'll add more detail in a comment.