The term 'ADD SUPPLEMENTARY MATERIAL' can be abbreviated to 'APPEND' (6 letters). This means to add additional material to a document, such as a book, article, or report. It is often used to provide more information or to clarify a point. Supplementary material can include illustrations, diagrams, tables, charts, and other visuals. It can also include additional text, such as footnotes, appendices, or bibliographies.