All appendix matter must start with either a paragraph styled as an appendix head, or with a figure or table that has: (a) "Appendix" in the label (e.g. "Appendix 1"), or (b) an uppercase letter preceding the number (e.g. "Table A1").
Depending on your eXtyles configuration, appendices may require distinct heading and paragraph styles. In these cases, if regular heading or paragraph styles are used inside an appendix, this may lead to a parsing error or to incorrect XML.