Add-on and Tag

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

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

Tag noun - A slip (as of paper or cloth) that is attached to something to identify or describe it.

Add-on and tag are semantically related in addition topic. Sometimes you can use "Add-on" instead a noun "Tag".