Add-on and Top-up

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

Top-up and add-on are semantically related. In some cases you can use "Top-up" instead an adjective phrase "Add-on".

Top-up noun - An amount needed to restore something to its former level.

Add-on and top-up are semantically related. Sometimes you can use "Add-on" instead an adjective phrase "Top-up".