Add-on and Addendum

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

Addendum and add-on are semantically related In some cases you can use "Addendum" instead a noun phrase "Add-on".

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

Add-on and addendum are semantically related in adjunct topic. You can use "Add-on" instead a noun "Addendum".