The unknown element is an open extension that allows information architects to incorporate xml fragments that do not necessarily fit into an existing DITA use case. Processors should ignore this element unless otherwise instructed.
See appendix for information about this element in OASIS document type shells.
This example features a specialized unknown element that includes other non-DITA content. If this specialization is imported to a DTD or schema, the DTD or schema will need to handle declaring the new elements or any namespaces.
<body> <my-unknown class="+ topic/unknown mything/my-unknown "> <thing value="4"/> <otherthing value="16"/> </my-unknown> </body>