Subdocument file entities are only supported in SGML documents. Although you can reference a complete document as a subdocument file entity, only do so if you know you need to include a file as a subdocument file entity. The SGML Declaration for your document type must permit SUBDOC. The subdocument file entity can have the same or a different document type than your document.

To create a subdocument file entity, follow the standard procedure for creating a file entity, and check the Subdocument check box.