Content model for the back matter (<back>) of the document, for example, ancillary material such as appendices
<!ENTITY % back-model "(label?, title*, (%back-only.class; | %both-front-back.class; | %front-back.class; | %sec.class; | %sig-block.class;)+ )" >
(label?, title*, (app | app-group | floats-group | index | index-group | ref-list | ack | bio | dedication | fn-group | glossary | toc | toc-group | notes | sec | sig-block)+ )