Add-on and Spin-off

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

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

Spin-off noun - Something that naturally develops or is developed from something else.

Add-on and spin-off are semantically related. Sometimes you can use "Add-on" instead a noun phrase "Spin-off".