An appendix is a section at the end of an academic text where you include extra information that doesn't fit into the main text. The plural of appendix is “appendices.”

Do you say appendix or appendices?

“Both are correct plural forms for appendix, but appendixes is preferable outside scientific contexts […] “Appendix typically has the plural appendixes in the anatomical sense, and appendices when referring to a part of a book or document.”

What are appendices used for?

An appendix contains supplementary material that is not an essential part of the text itself but which may be helpful in providing a more comprehensive understanding of the research problem or it is information that is too cumbersome to be included in the body of the paper.