This defines games or utilities available separately that needed another game to use. For example, the game “Antheads: It Came from the Desert 2” requires the original “It Came from the Desert” disks and would therefore be defined as an addon.
Another example of a game addon, and one you could get for free by registering your copy of Skidmarks, is the Skidmarks Racer Magazine. It contained a A4 booklet and a disk with an HD Install utility, extra cars, etc.
Addons may have later been included with a release of the game, like the data disks for Test Drive 2.