The official add-ons are PWADs that can be downloaded from Bethesda.net to be played with the Doom Classic Unity port. Since console manufacturers do not usually allow players to use third-party content unless vetted by the publisher of the game, the official add-ons system allows a selection of modifications to be downloaded and played instead of being limited to the IWADs. Official add-ons are curated to ensure they are playable within the raised limits of Doom Classic, and to avoid potential copyright infringement in their content. Also, the consent of all original authors is mandatory, so that they all have to be contactable by id Software in order to license their work.
The two Final Doom IWADs are available as official add-ons for both The Ultimate Doom and Doom II. This effectively makes Final Doom free for all owners of either Classic Doom titles on Bethesda.net. Thanks to DMAPINFO, some of the curated PWADs can also be made available to both games.
Note that "official" in this context refers to provision of the content by id Software. There has been no statement indicating that the modifications created by the community should be considered canonical story-wise.
List of official add-ons
|Title Image||Name||Game||Authors||Original release||Made available|
|BTSX Episode 1||Doom II||Esselfortium et al.||2012-12-10||2020-06-25|
|Double Impact||Both||Ralphis & RottKing||2011-11-30||2020-02-13|
|No End In Sight||Doom||Emil Brundage, Christopher Lutz, Xaser||2016-11-20||2020-03-31|
|No Rest for the Living||Doom II||Nerve Software||2010-05-26||2020-01-09|
|The Plutonia Experiment||Both||Dario Casali & Milo Casali||1996-06-17||2020-01-09|
|TNT: Evilution||Both||Team TNT||1996-06-17||2020-01-09|