Addendum: Definition

Document containing official additions to a game’s rules, made after its publication, in order to clarify, adjust or expand them.

From the Latin verb addere (to add), addendum (plural addenda) means “that which is to be added” and has been used since the 17th c. to designate passages that were to be added to a published book. Nowadays any type of document, from a contract to a podcast episode, can receive an addendum. The plural addenda is used indifferently, but seems more scarce in the game world. And like errata, the plural form can be used as a singular (e.g. an addenda has been posted).

This Addendum brings a variety of additions and options that can be implemented in your games to make it more “complete” or aligned with the tastes of whoever is playing.DriveThruRPG

I notice that the Caverns of Bane Rules have an addendum for the Solo Rules.Chris Lawson

Just like an errata, an addendum to a game is made or at least validated by its designer or publisher, and is considered to be part of its official rules. It is also sometimes distributed along with the game, as a sheet insterted in the box or the rulebook. The new edition of a game will normally include all addenda to the previous editions.

Clarifications and FAQs differ from an addendum in the sense that they usually further explain some existing rules, whereas an addendum typically modifies existing rules or adds new rules and options.

What can you add to a game’s rules? Well, new scenarios, new characters, solo or team rules or other variants.

Errata can be seen a specific form of addenda, which contain corrections to rule errors discovered in the published rulebook. Addenda are rather meant to cover game play situations not provided for by the published rules, or offer new material.

The term addendum is sometimes used colloquially as a synonym of extension, companion or spin-off game, or to designate any additional content pertaining to a game, or a volume in a game series. For example: “Race for the Galaxy, and its acclaimed addendum, Roll for the Galaxy”.