Add-on and Fixture

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

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

Fixture noun - A person who has long been associated with a place or activity.

Add-on and fixture are semantically related. Sometimes you can use "Add-on" instead a noun "Fixture".