Add-on and Appendix

Add-on noun - Something that is not necessary in itself but adds to the convenience or performance of the main piece of equipment.

Appendix and add-on are semantically related in addition topic. In some cases you can use "Appendix" instead a noun phrase "Add-on". popular alternative

Appendix noun - A part added at the end of a book or periodical.

Add-on and appendix are semantically related in addition topic. You can use "Add-on" instead a noun "Appendix", if it concerns topics such as supplement. popular alternative