Excerpt information may be written directly to the Resource document. However, if you write excerpts for a book, you may want to group this information by topics discussed by the author.

The Excerpt doctype allows to place this information easily.

The lifetime of subdocuments is associated with their parent documents. That is: if the parent document is removed, all subdocuments will also be removed. If a document stands for its own and should outlive the referencing document, it should be added as a root document to a doctype homepage.