Add-ons or expansions (also called expansion packs) are content created for games that is released separately from the main game release. Downloadable content (DLC) is a specific form of add-on distributed through the internet.
Fallout: New Vegas
Fallout: New Vegas also featured add-ons, with a total of four pre-order bonuses, four story add-ons and two items packs.
Fallout 4 released six add-ons in total, half of which were story add-ons, while the other half primarily expanded on the workshop settlement feature.
Fallout 76 is expected to contain add-ons in the form of free expansions after the game's release.